Lawrie, I considered a 3.54 gear set with my Burtz engine and decided to wait. I'm happy I did. In fact, I am beginning to question whether I need the O/D at all with the new engine. It revs so freely and smoothly that if I were cruising at say, 55 mph in O/D, the car is now happy in ordinary 3rd gear and "singing" in a higher key at that speed. I have a virtually new 3.54 set on the shelf and if I don't use them in another of my cars, they will likely sit there for who knows how long.
How strong an engine? There have been a couple of times when I have forgotten to take the car out of O/D when I stopped and taken off in high first with the camper behind. Sure, it didn't make my cheeks go baggy when I pressed the accelerator but it did it. That engine was not as powerful as the one in your P/U but might have had more grunt t the bottom end.
A 3.54 diff and O/D is a pretty high ratio - about the same as using 5.25X21 tyres on the rear of a 3.78 diff. I run that on my tourer and it gallops along. Your engine will handle it easily.
