Makes sense. I assumed an A-body. That loud noise you hear from knobby tires is energy being wasted. Shift to "N" and see how fast it slows from 65 mph. If it keeps decelerating the same after 40 mph, the drag is more from the tires than aero.
The tires definitely have drag. Back when I had an XJ Cherokee I put these same tires on it and it went from averaging 20 mpg down to 17-18.
I will say I drove the truck yesterday and I'm not convinced the ported vacuum advance is the way to go. Performance was fine but when idling in traffic (evening rush hour) the coolant temp would climb where before with MVA it didn't do that. I am for sure going to redo the mech advance curve and try swapping the pickup leads.