I should preface this by saying that I know very little about drivetrains.
I believe that Toyota drivetrains of the mid '80s were stronger than contemporary -- for instance, Toyota used an 8" rear diff while Ford put a 6" rear on the early Rangers. However, In my ignorance I worry that with an engine swap, either the driver would have to be careful not to damage the drivetrain (thereby manually limiting the power output of the engine and the pickup as a whole) or they could be un-careful and break the weakest link that was only designed to perform reliably at, say, 120% of 22R specs, thereby still limiting the pickup as a whole to close to what its original engine would do. Why, for instance did Toyota build a stronger transmission for each new generation of engine? Perhaps because they didn't believe the older ones would hold up well enough?
That said, there could be many other reasons to swap an engine. Parts availability, fuel economy, different torque curve, etc. If I thought I could do a good job on a 3RZ swap, I probably would, just for the better efficiency and the addition of fuel injection.
OK -- my $0.02 has become $2. Let's get back to your truck.
