cooling my slant 6

As above... what year? We can't give the right info without that.

With a non-overflow system, the coolant should be filled to about 1" or 1-1/4" below the bottom of the filler neck. This is specified in the FSM for the older, non-overflow cooling systems. Filling higher will just result in overflow and blowing out 'til it gets low enough.

If you DO have a coolant overflow, and the coolant is always low, then you could have some issue with the radiator cap, or even possibly a coolant leak into the engine.

How about hooking up your temp gauge! Cruising at 70 will show higher temps than cruising at 55 so don't be surprised if it goes up a bit on the interstate.