A couple of reasons.
First the cost of the 4l80e and related stuff new was going to be three or four thousand against using a BOPC TH400 I already had. I don't expect to use the finished truck enough that it would pay back that investment in any reasonable time.
Yeah buying the transmission used could save much of that, but even looking at just the cost of the computer with harness and the Cadillac adapter the conclusion would be the same. And buying a used transmission risks needing to rebuild one, and the 4l80e isn't as easy as something like the TH400.
And second I plan to use this truck to pull my trailer and it will rarely go far without a load behind it. The combination of tires and rear gears put the motor in the ideal RPM range at freeway speeds already.
An OD tranny doesn't gain me anything with the big Cadillac motor in my application. If this truck was to be a daily driver I might make a difference choice, but I don't think it makes sense based on my current plans.
