Generally speaking the legal aspects of swapping a newer (cleaner, in theory anyway) engine into an older chassis is pretty easy, even in cali, as i understand it. Logistically, however.. whoooooooolleeee different ballgame.
I'm currently putting my 94 frame/drivetrain/suspension under a 97 body, so I have a bit of experience on the subject. So far I'm at something like 4 dozen solder joints.
First off, the 91-92 chassis/body will not have the pigtails on the harness for the A442F transmission computer. These two pigtails are wired into the dash harness near the driver's footwell. It wouldn't actually be extremely difficult to put these pigtails near the ECU, but realistically compared to finding a similar 93-94? WORLDS different amount of labor involved.
The engine part itself is more of an unknown to me.. but I can say that the 1FZ ECU interfaces with the body harness for a few things (fuel pump resistor control; air flow meter partly controls the circuit opening relay which provides power to the fuel pump; tach signal actually bypasses the ecu and comes from the coil, which is very different on the 97 requiring some trickery like the circuit opening relay); other stuff I'm forgetting)
Not to mention the logistics of actually swapping the body. If it weren't for my friend's forklift, big parking lot, and fabrication shop it would have been a matter of finding a shop with TWO spare 2-post lifts and begging for time on them.
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Basically, take it from someone currently dealing with your exact scenario. I bought a 94, spent months going through the engine fixing everything (literally no oil leaks!!) so when I rolled it a few months later and the roll didn't damage any suspension/steering I thought I had big incentive to keep that drivetrain and find a donor shell.
Realistically if I had just waited for a good body/frame to swap my lockers/running gear into I could have paid for a significant chunk of the vortec swap I want in money/time.. and not have to do dozens of hours of wiring to get my pig running so I can (salvage/rebuilt) title it before I rip all the damn electrical stuff out for said vortec next year!! All in a hodgepodge body with a salvage title!
Yeah. Don't bother. Find a same-year donor or just keep the crap you want.
I'm going to bed.