Try here:
http://www.davessmallbodyheis.com
He can take your existing Points dist. and convert it to an electronic ignition. That way you don't have to worry about fitting the larger body of a stock GM HEI in there.
Plus, I've heard really good things about his conversions...
Otherwise, find a HEI out of a 74 455 and install it, just run a 12volt wire in place of the resistor wire you have right now. You can get replacement wires from
Http://www.TAperformance.com That'll just plug into your existing wireing harness.
Have fun.