There are a BUTTLOAD of aftermarket racing parts for the 6.5's.. You can even change out the crank, do some machining, get a short rod and make a stroker motor out of it..
I'd just throw a billet rod and hopped up cam, stronger push rods, better carb and pipe..
Theres lots of stuff you can get for them pretty cheap, as many people run them.. Briggs even builds a factory racing engine from this platform World Formula..