No need to provide pics. I understand now. I was thinking what I would do next. I might take out all the plugs to see if the starter really starts to spin. That would still put it under some load to rotate the mass and could give some clue. I may also try pushing it or pulling it in 2nd or 3dr to get it started. I may check timing before any of that. I may start to second guess my starter rebuild after that. Putting a volt meter on battery while trying start motor may also give a clue depending what static voltage is and what it drops to when cranking with plugs in and plugs out.
I guess your battery is in the floor and the trans cover is near the battery post, but not sure if there is grease, oil, gasket, paint involved in getting ground to the starter motor body from there. Maybe a direct cable from battery to starter mounting bolt would give a clue. Right now the issue is starter speed, so with a foot start, the only ground that matters is the battery getting good ground to the starter metal. A bench test hook up while on the truck.
Make sure the starter mounted button switch is good and it's copper landing pad on the starter is protruding enough and clean. Depress the starter button by hand or 2x4 to eliminate linkage throw problems and to be sure it's not a partial button contact issue. Check oil. Loosen belts. Any drag on motor issue you can think of. Trying to convict the starter.
Anyway there is a bag of thoughts for you next weekend.
