High-quality machine.
Expensive to rebuild.
I don't know about upgrades; if I wanted to do this I would put a crate GM 350 in it.
What condition is the engine in? I would evaluate: compression? definately new timing chain, tensioner, rails if it hasn't already had them. The basic engine might still be solid enough for more service with just heavy maintenance. I would not run it hard unless I either did not care what the likely consequences were, or had plenty of money to fix what breaks. Good luck.