what is a safe and logical RWHP I could establish with a 350 engine and a 700r4.
If you don't have experience building high horsepower motors I'd suggest investigating the crate motor options. Pick up any hot rod oriented magazine and shop the ads.
600 horse is a lot of power but can be bought easily if you have the money. A 700r4 in stock trim isn't up to that much, but again modified versions just cost money. If you have a $20,000 budget for the engine and transmission swap the horsepower part is easy.
But look at the cars that come with that kind of power rating and think about the safety features including brakes. The brakes on your '59 are woefully inadequate for the kind of power you're talking about.
Your first step should be to make sure you are not going to kill yourself or someone I care about. Please look into making the suspension work work and to adding excellent and adequate brakes before dropping 600 horse under the hood.
