FORGET the old heavy stuff. Dropping a 700 lb 440 or 800 lb Hemi in the front end would kill it for driving.

It's not that heavy a car. Build it as a driver with a modern 5.7 Hemi and either 5-speed or OD auto. Keep the fuel injection and use something like a 3.55 rear and it'll be too good to be true. Shoot, a warmed over 5.2 Mag engine with A500 would get the job done. But I've got a burnin' in my belly for a Hemi swap that makes great power with economy so it can be driven hundreds of miles. After that is all put together you have to install air conditioning.
Also, use the stock frame with an aftermarket Mustang II suspension and rack-and-pinion steering. Fatman Fabrications or one of the million others you can find. Gotta have tubular A-arms.
I could go on for days planning this car.....
R.