While I agree it's probably not the best for gasoline, your 99 pump is also listed for the inter-tank transfer pump on 5.7 and 7.4 trucks with dual fuel tanks.
It looks from GM the mirror side connector or harness isn't available separately, but the bodyside connector is used in many applications, including many later-model seat heater systems, so your options for scrounging are better than if you were limited to these bodystyle trucks. Personally I'd...
Then you need 15644903 which unfortunately is discontinued, and nobody shows stock at this time. I guess aftermarket or used are the only options, or modifying one to work.
Water pump plates:
12562637 is the number for up to around 1991 in most applications, I assume with v belts; 12562659 is used for around 1991 and newer 6.2 and 6.5, again I assume that one is for serpentine belt setups.
No personal experience here; I guess I'd look at the molding lines on the backside of the pad to try and tell if it was molded around the pedal arm or if the opening is oversized enough to fit over the pedal. Or if your 96 is nearby to compare, it should have the non-removeable pad.
You may need to remove an orifice tube from the condenser; GM condensers have one installed that is only for front-air trucks. Subs with rear air should only have an orifice tube in the Y pipe.
Subject: 93 - 00 Full Size Utilities (GMT400 Old Style Only) AC Blows Warm and the High Side...
OK, it sounds more like just the rear side door lock actuators crapped out, which is fairly common (although not 2 at one time). How to get the doors open for repair I don't know.