On the RF side of the frame is the VIN of the frame, just behind the RF wheel/axle.
In the cab, the VIN for the body will be both in the left base of the windshield and on a door sticker.
That may be worthwhile to gather both of those pieces of information so they can be cross-referenced...
This is a 1992 truck swapped with I'm guessing an OBD1 (93-94) 1FZ-FE.
Doesn't the wiring on that relay box have frequent problems on the 91-92 due to wire/crimp issues on the bottom of the box?
I realize this truck is a Frankenstein, but we have to look at the multiple issues it may have...