When You say power pack heads are you reffering to the heads that would have come on a 283 --4bbl carb or are you reffering to the 462 heads that would have come on a 66 --275hp--327 .Why not pull a valve cover and get a part# and build date off the head. This will help to arrive at a much more accurate estimate of horsepower.If its the 462 they will have double humps on the front of the head. A 750 sounds like too much carb. either way. It would proberly run better and faster with a smaller carb.600 cfm maybe.