assuming those are 195/70-14 tires, your RPM's in 4th gear (1:1 ratio) should be just shy of 3000 at 65. Not bad, but yeah it'll be loud.
Personally, I think your carb tuning is probably too rich at the load/speed combo you are cruising at in OD. 3G Camaros came stock with 2.73's and a TH700-4R, giving a final drive of less than 2! You're talking 70mph at under 2000 rpm there.
Now, accounting for the Nova body being less aerodynamic, and the smaller engine, your current combo (assuming a 0.73 OD) still puts you at about 2175 rpm at 65, which shouldn't be too much for the engine.
What I expect is happening is that you have to have a fairly high throttle angle to cruise that way, and that is dropping your vacuum down low enough to trigger the carbs power circuit. Some carb retuning with a stiffer power piston spring ought to get that mileage back, but you'd really want to have an A/F gauge to feel safe.
Another thought: is the vacuum advance on ported or full manifold? If you're not getting vcuum advance at cruising speed, that would cut your mileage down.
