Slanty, 904 issue

Ive searched the forum and cant find anything. I have a 71 Duster. Original 225 and i believe original 904 and 7 1/4. I'm not sure on the rear end gearing. But i can barely get to 70mph on the highway. The engine is running hard as it can. No tach so i don't know the rpms. Ive used a gear calculator to check different rear-end combos from factory and none seem to be fitting that kind of highway speed. Im guessing the slant won turn over 4000-4500. Tires are 215/70 14.
I need better highway speed without setting off a grenade in the engine. Does that speed soud normal? Any input or suggestions?