That kind of quote sounds way too high. Maybe you shouldn't have mentioned the name "Avanti" when getting a quote! I know when you attach the word "Corvette" to auto parts needed the price goes up, so this may be the same thing.
Radiator shops generally order cores by dimensions and whether cross-flow or vertical-flow, not application, unless they're ordering an entire radiator. For a given size core the labor costs should be the same, regardless of what kind of car it came out of. That assumes you're carrying in the radiator to the shop, not letting them do the R&R in the car itself. If they do the R&R then costs will go maybe as high as the quotes.
If you order a core, ask for one with more cooling fins per inch, if one is available. That will increase cooling efficiency without increasing core dimensions.
That temp probe needs to be installed in a cooling passage...either one in the radiator or the engine...not sure of where since I'm not familiar with them. Maybe the thermostat housing, but I don't know. It's not doing any good just hanging there. It could be your radiator is leaking from where the temp probe should go in?