I'll have to agree that the light duty Cummins diesel thing was also a little bit of a letdown to me when I really looked into it. Still cool, but not nearly as cool as it looked at a glance.
TJ; FWIW, your truck should have 3.73 gears. 3.90 was the earlier (pre '63) Eaton ratio. Could have 4.11's, but I doubt it.
I'll also add that IME/IMO 4,000RPM is a really excessive shift point for normal driving with the SM420/inline six. I have a weaker 250 in my one ton and normally shift @ something like 2,200-2,500RPM (most of the time). It's certainly not necessary.
FWIW, 292 GM spec is (net) 153HP @ 3,600RPM and 255ftlbs @ 2,400RPM in '65. (I'll post the dyno sheet as an attachment).
So, (in stock form) the 4BT and 292 will probably pull similarly up to 2,000RPM or so, but the 292 will have another 50HP available higher up. That ability to wind it up a little and get half again as much power is kind of nice...
