Just slap a Pontiac 400 in that bad boy. Hop on CL, look around for a good ol block outta somewhere (you can find a real decent used one to rebuild for less than $1000) The car's body was made for a 400 Pontiac so it'll fit no probs. Unless you're building a 600 HP engine, a TH350 will be fine right? Cheap and easy to rebuild. It's all about slapping, really. Slap the engine in, slap together a TH350 rebuild kit, done.