Not sure if you have ever driven a F150 Ecoboost? No disrespect intended at all here, but the empty weight of a full size 2019 f150 is 4069 lbs., and with the ecoboost engine these trucks are no dogs at all. Not very much weight difference between the 66 Tbird and the truck. With the right gearing it would make the 66 a nice performer, and fuel mileage a lot better the the 390 or 428. I'd say go for it, would be a neat project and the thing about these Tbirds is they don't demand any real high collector prices anyway.
Just my opinion here and no disrespect of any one elses.
Take care
Ed