Iron Clad, I'm suprised yours keeps up on the highway just fine...does it have stock rear end gearing too? Mine has the stock 4 speed and the 3:90 gears, it'll do 60-65 miles an hour...if there's no wind, and it's flat ground. If there's wind, a hill, or both, I have to floor it and it wont go over 55-60. Everyone does drive really fast here in Alberta though.
I've been thinking about what you've said here, and it reminded me of a very similar performance issue I had. Flat road, no wind, it would go just fine. However, a headwind (or even the lousy aerodynamics of these old brick shaped trucks) and/or any noticable grade would slow it down considarably. I would have to mat it just to keep a decent road speed, even in overdrive. Ended up being the clutch. I had the pedal linkage too tight, so it was basically operating like as if I was riding the clutch all the time, making it slip. Readjusted the linkage to the spec of 3/4" freeplay, problem solved. I can easily run with pretty much anything on the road. 70+ is no problem in 3rd, OD locked out. In overdrive, well, lets just say I've had it way faster than anyone should really go with stock brakes, it'll run 75-80 at 2600-2700 rpm.. I'm running the same 3.90 rear you have, with a 30" tire. My engine is warmed over, with dual Holleys, Fentons, milled head, Howar M4F cam, and upgraded ignition (Mallory, not Pertronix).
