I casually mentioned testing for power on pin 30 at the relay in my first post in this thread as means of starting the terminal identification as this pin should have +12v at all times. If you had to run a jumper, you would want to find the purple wire, pin D11, in connector C2 - Black under the underhood fuse/relay box. That would be where you want to tie your replacement wire in.

But I see that you said placing a jumper between pins 30 & 87 works which proves out the wiring from the relay to the starter as well as the input side of the relay. If your other testing proves true, then that would point to a bad relay. You can test this by swapping it with another 4-pin relay in a different location and seeing if the truck will turn over.

If swapping in a known good relay still doesn't allow the starter to spin over, then one of your previous test results has to be fishy since your jumper from pins 30 to 87 indicate a good input & output from the relay switched contacts. You tested the coil side inputs for +12V when the key is in the START position and that there is ground at the terminal on the opposite corner of the relay socket. Maybe verify those results?

It shouldn't matter what engine is in it as the controls are very similar if it is still an SCFI Vortec engine (or possibly one upgraded to MFI). The 4.3L was just a 5.7L with 2 less cylinders (more or less). The SCFI Vortec 5.7L was the easiest to swap into the 2nd gens as it involved adding the wires for the injectors, lengthening a few harnesses to reach new sensor locations, and a PCM reprogram on the electrical & controls side (probably over simplified, but whatever).

But you said that you have power & ground to the coil side of the relay (pins 85 & 86). That would indicate that the ignition is doing what it is supposed to do. Is that not the case?

If it is the case and proper power & ground are getting to the relay coils to energize it and when jumpered from pins 30 to 87 the starter turns over and if a known good relay is used and still no starter, then the only remaining possibility is bad terminals in the fuse box I guess.. That is strange, but if somehow a pin was forced in wrong, it could have bent a terminal inside the fuse box and caused an intermittent issue with the connection between the fuse box & the relay itself...

It isn't easy, but the fuse box can be disassembled and checked for damage. There are A LOT of clips in a deep lip around the outside of the upper & lower halves that all need to be disengaged (of course this is after you remove all of the relays/fuses & disconnect all of the connectors on the bottom). Once the upper & lower halves are separated, you should find multiple layers of metal circuit traces & plastic that make up the various circuits from the connectors up to the fuse/relays and back down to the connectors. Fairly simple, but DO NOT BEND ANYTHING! You'll have a bitch of a time getting it back together if things get bent out of shape.

You can grab a complete fuse/relay box out of the junkyard, but I believe there are two (or more) different underhood fuse box configurations. If I remember correctly there is a base and uplevel design to it. Just make sure that it has all of the same locations and terminals as your current box does and you should be set.

If you are getting all of the appropriate power & ground signals to the box, then that leaves the box as the problem. Odd as it may be, but that is why you test things out and by testing at the relay itself, you have verified everything to be working at the easiest location possible.