I agree. You probably should had hit 3rd Hi or even 3rd Lo before the big drop off. I used to haul a Full Cord of wood with my '64 C-30 with a 292 and always hit the same long, steep hill on the way back. I would keep it in 4th till the RPM's dropped off at a certain point, then quickly double clutch downshift to 3rd. On this hill I was able to get a running start at about 60, but never dropped below 30 at any point.
I'm not really familiar with your truck, but a Straight-Thru Muffler makes a pretty big difference for some extra power.
Makes it sound sweet too !!
Bill
