Since I'm building (or should I say trying to build, once I get more
to finish) a 70 ProStreet Duster, if I was to do it again I would look for a Roller/ ex race car/ProStreet that is already pretty much done, then just drop in a motor and trans. It will save you a ton of money.
ProStreet Cars are selling pretty cheap, right now on CraigsList in California there's a nice looking running ProStreet Demon with a 440 for like $12,500. Once you add up the cost of a narrowed axle, suspension, tubs, rims, tires, roll cage, frame work, and the labor to put it all together, it adds up very quickly.
Ever thought about trying to trade for a Tubbed E body? That way you still get what you want, without chopping one up.
Good luck, I love ProStreet cars, I just wish I had one that was done!