You just need an 8mm or 5/16" Box-End wrench for the stock side terminal bolts. This is the one size that's so close that either wrench will work.
Ratcheting Box End wrenches are nice to have but not completely necessary. I actually prefer 6 point wrenches in this area.
The wiring to the fuse panel terminates in Pak Con connectors. You can purchase individuals of these terminals from Mouser and Del City but you'll need decent crimping tools. They are not cheap. The below tools used to be in the $89 range but they are up around $135 now.
They are very versatile tools... You can do most if not all of the terminals on these trucks and many other vehicles with those two tools. The exception would be the oddball double wire Packard & Pak-Con connectors. I would hazard a guess that you could re-sell them on evilbay for $120 and they would go fairly quickly.
12085271 is the Delphi Ratcheting tool for 14-20AWG Packard/Unsealed-Metripak/Pak-Con
12085270 is the Delphi Ratcheting tool for 10-12AWG Packard/Pak-Con/Sealed Metripak & Weatherpak connectors.
Older glow plugs...
If you have the round glow timer/thermal-switch on the front of the intake manifold they tended to get dodgy. The connector is GM 12004933. It's obsolete and a quick web search returns no information on it.
The round timer/switch and contactor can be updated to the more reliable controller and contactor one-piece control unit that GM used on the 6.2 & 6.5 diesels from 1985 through my 2000 GMC2500 that was built near the end of production of the 6.5L in the T400 chassis. The glow controller wiring page in the 1989 RV series wiring manual is easier to follow than the 1985-1988 wiring books. They are all wired the same.
The five terminal Metripak plug is labeled A-E.
-A is empty
-B is wired to the CRANK wire on the starter. I usually just fish a purple wire through the steel sleeve on the bellhousing and put a #8 ring terminal on the end so I can stack it under the stock CRANK wire on the #8 starter S terminal. If it's a #8 ring terminal then it can't fit on any other studs on the starter solenoid.
-C is from the temperature Inhibit switch
-D is tied to the same ignition switched circuit as the injection pump fuel cutoff. I used a piggyback terminal on the injection pump cutoff solenoid.
-E is Ground.
Piggyback terminal... McMaster Carr has these but they're all over the place...
Glow inhibit NOTE:
You can leave the inhibit switch out but it will glow when the engine temp is high enough to not need them. The wiring is self explanatory. Usually the inhibit switch is installed in the cylinder head... IIRC I just replaced the temperature gauge bung in the RH cylinder head with the inhibit switch. It's a normally closed thermal switch that opens when the engine temp reaches 125°F.
You may be able to just add a piggyback terminal to the HPCA (Housing Pressure Cold Advance) on the injection pump and run a single wire from there to terminal C. From the looks of things that'll accomplish the same thing as adding yet another thermal sensor. The difference is the HPCA thermal switch opens at 95-100°F. I didn't have the wiring diagrams when I updated my 1980 350 Diesel glow plugs or I would've used the HPCA power to the inhibit terminal.