The powerglide if built will hold up. But have to be built with different performance parts. If the car is heavier then you need a built TH350 or 400 turbo.
The heads of choice as far stock heads are the 049's an 781's. There could be some other heads out there that would work also, but time has proven what flows better. The two numbers I have listed are a good start.
If you go with a 4.250 bore then you have a good starter block. The 3.76 stroke will net you a 427 an the 4.00 stroke will get you a 454. The 454 combo with .060 bore makes for a puller at 468 cubic inches.