My opinion.....Using the window openings puts a lot of stress on the roof / The only steel in the body itself, are the Torque Boxes (TB's)(hog troughs), which are connected in the front, to a pressed steel frame around the A-pillar that supports the windshield. The rear of the TB's are bolted to the roll bar, so there is NO steel support between the windshield and the roll bar. The fiberglass roof is only ONE PLY of fiberglass or about 1/8". It's layered and compounded at the weatherseal areas, but... See this photo of a burned out avanti, exposing the steel in the roof. The body itself with the doors, windows, etc, weighs about 800 Lbs. You could risk popping the windshield out or worse, screwing up the roofline. Granted, the more weight you remove, the less the effect, but ..... your choice
When I lifted the body off my car, this was the process and is actually a test of my memory, but it's pretty accurate and one man can do this.
I pulled my car into the garage ( 26'x28') and jockeyed it diagonally with floor jacks. Today they have those wheel dolly's that would be great but they raise the car a bit higher, initially.
NOTE: Frozen bolts... The TB's are not only body supports, but the insides are subjected to any water, rain or otherwise that enters the grill below the windshield, runs down the duct at the kickpanel vent and into the TB's. Avanti II's that have moonroofs, have a gutter drain tube that is detrimentally routed down the roll bar pillar and into the TB's also. So, many of us older owners were aware of this early on and saw fit to drill drain holes in the bottom of the TB's. Further, we would occasionally plug the drains and pour a couple quarts of used engine oil into each side through the kickpanel orifice after removing the grille. After a few days of letting this oil slosh around, the drains were again opened and the oil removed. This treatment was paramount in maintaining the integrity of these boxes. You could also inspect the TBS's by removing the sill plates and by using a 2"+ hole saw, cut a hole every 6" or so. These were covered by the trim, so no one will know they were there. So, if your TB bolts, which are SAE thread at 5/16:, I believe, are rusted in place and spraying them with a good rust displacing oil ( PB Blaster, Kroil, etc) does not work, try cutting the access holes and attack them from the top. I can guarantee you it beats having to drill them out.
The body bolts are as follows... There are 2 bolts at the radiator core support, then the two at the firewall (accessible from inside the car on the forward floor, where there are 2 circular covers that expose the bolt) Remove the firewall bolts completely / then loosen two at the same plane in the TB's, 3-4 under the TB's on each side, then completely remove the 2 after the rear wheel arch inside the raised area attached to the bumper, but loosen and leave the one that bolts the trunk support to the spare tire well. Do not remove them entirely, except as stated. Use this as a reference.. Here are the TB's
At EACH body mount point, COUNT, NOTE and MARK the body spacers as once you start the body up, they'll fall out and things can get confusing. These shims are not all the same thickness and have to be reassembled as they were installed.
Before I began the lift, I started by making pylons of two concrete blocks outside the ends of the TB's a foot away from the car, then jacked the car up by the frame as high as it could go. The front by the center of the main crossmember, the rear by the differential (No safety stands this time) / Remember the body is now only attached by the bolts at 75% looseness /
Place a stack of 12x12" sections under a point about 6" from the end of of each TB, stacked up as high as they will go / You should have wheel arch, 6" of TB (No more) , then the stack of wood on each corner / Let the jacks down slowly until the TB's rest on the wooden stacks / At that point, going further will start to separate the chassis from the body / This is the point of no return / Have your body mount bags ready for those steel spacer shims. I cannot stress this enough, you cannot confuse their sequence or remounting the body will be a disaster.
Lowering the chassis further now, will start the separation, so go very slow / You expect to see the chassis moving away from the body and the heads of the remaining bolts that were at 75% looseness, should start disappearing upward. Continue slowly and monitor the bolt heads. Any that don't move, means you forgot to loosen something or something is stuck. The body mount shims have rubber isolators which may stick to the chassis or body and abruptly fall off mixing up the spacers, so be aware of the above paragraph/
Once you are SURE there's no interference in the descent, remove the remainder of the body bolts entirely. the chassis is on the ground and things are clear, The TB's are at least a foot off the floor / move the jacks to the ends of the TB's, put a large piece of cloth wrapped wood between the jack pad and metal TB and pump it up and inch or two / Slide a piece of 2x12 into the wooden stack on each side then repeat this continually, on the other corners until the body is high enough to slide a couple 8 foot, 4x4"s or 6X6" under the body and above the chassis. The beam ends are going to supported the front and rear of each TB and the beam ends will sit on the concrete blocks. When the beam is finally placed under the body, they will likely need need some height to support the body, so insert those wooden 2x12' pieces under them / Build up the wood shims under the beams until the body is supported by the beams on the concrete blocks and you can start to remove the wooden stacks between the floor and the TB's.
Once the body was supported by the wooden beams under the TB's, you can easily jack up the outer end of each beam and slide another 2x12" shim to the stack and slowly raise the body / Continue this rota and once the height of the wooden shims is greater than the height of a cinder block, then remove the wooden shims and replace them with another course of cinderblocks. Continue this rota until you have the body as high as you need.
I had mine high enough (4 feet+) to roll the chassis out and do the necessary work on it, replace the engine and transmission, etc. So nice to be able to work on the chassis by looking down at it..
The body at 4 feet or so was also great to work on, stripping the lower areas of paint and repairing any damage.