As its an easy swap for now, I would for sure try an 850/950DP if he could borrow one or has one, also try a 1" spacer as well, I think it could pick up 2/10ths, just like my little 340 did with stock heads from a 750>850DP....be interesting to see, could be even more?....think Mopar V8, think big carbs especially with 4.56/5000 stall and a victor intake, 750 is puny in my book with that set up....the larger hdrs/heads can come later...squeeze what you can first with what you have and easy bolt-ons/tuning, the chassis is already good....could shift it a tad more than 6k though I think with 256/268 @ 50 dur and see if it picks up mph which it should with the bigger carb.