I have been meshing the electrical of an '88 to the frame of my '85...
'88 has one big connector going to the back for the tail lights, fuel sender, dome light etc. '85 does the same thing with two smaller connectors. PO mine did this adaptation with a jumper harness full of butt connectors. I took both apart and repinned it so now the rear harness uses the same bigger connector the '88 dash harness uses. At Carquest (I think BWD) the pin part numbers are female CG30 and male CG31. They will give you a blank look when you ask and will never retain it if you go back for more, I have them saved on my phone now.
I tapped into keyed power at C102 and ran a wire from there to the passenger side fenderwell. I have that running power from the battery to a fuse panel to run all my added crap that needs keyed power. Ignition and the choke might entertain you, BTW a factory carb doesn't get straight 12V, it will open early.
C102 and C118 should catch most of your major problems.
I have a lot more going on than you do as far as aftermarket accessories go but you can kinda get the gist of what I am saying with the added fuse panel.
If the '88 is an automatic it will be a one year only search for a clutch pedal, they were changed for '88 and uses its own clutch position switch.
4wd indicators changed too, older uses one wire and newer uses two. May run into differences with charging indicators if one is a idiot light and the other has an ammeter also.
Electronically '88 is a transition year, a lot of new stuff for the second gen in '89 was starting to get worked in. Not really major stuff but enough minor stuff to about drive a guy nuts trying to mix and match things together.
If you get into a tight spot drop me a line, I just went thru a lot of this earlier this year and I have both '85 and '88 EVTM's. They run around $15 a pop on ebay and will make your life a lot easier with your soon to be mutant truck. They have been a godsend on mine.