bigd1,
I can tell you what I know, which ain't a lot.
Your Magnum is probably around 5000 to 6000 ibs, guessing? The old magnums are heavy. Thats alot of weight for a single engine. More HP will get your more speed with the right prop configuration but if you are only turning your current prop at 4000, I would try re-proping to get in to the 4800 to 5000 range and see what the speed does. If your wanting to see 70ish, and my guess on your weight in close - your probably going to need to look into more HP.
What Stern Powr are you running? 101, 103, 105 or MX-07? Your can get internal hydro steering on the stern powrs, if you can find the parts.
I can tell you my old Cigarette 28 with twins, producing alittle over 800 HP total and running the Borg Warner trans and Stern Powr MX-07 drives has NOT ever seen 70!
69 mph - GPS is the best I can do at 5200 RPM.
Good Luck