Yep, fixing the overheating and sticking with a FE is your best bet IMO. I haven't dove in with my grandma's 64 Tbird yet, but I have done quite a bit of research. If you hop up the 390 too much you can crack the case on the stock trans. A C6 swap can be done reasonably easily. A guy on a Tbird forum showed pics of his C6 swap with a 65. X-member modifications, driveshaft and some fab work to mate the column shifter to the C6 linkage is required. If you do decide to build the 390 be ready to cough up $$$ for headers and watch hood clearance when you look at intakes. If you haven't discovered this already, there is a warehouse in West Linn, Oregon called "Bird Nest" that specializes in 2nd-4th gen Tbirds. Any Tbird part you need they have. If you google "tbird nest" their website will pop up.