What will fit / work?

One option would be buy a junkyard magnum 5.9 (360), use the 273 chain cover, waterpump etc, and invest in a set of EQ 318B heads. These are Magnum heads that are drilled for LA (340) intake manifolds, etc. You will need to put bronze guides in the new heads most likely, but for about a grand you can throw together a decent set of new heads, get the stock rollercam reground, and buy a cam extender ($40?) and eccentric to run a stock fuel pump. If you put decent exhaust on it, it ought to make 350 hp? Maybe more.