Danny is that a "C "after the P8E29 on the 4 speed? If so wouldn't it be a M-22 trans?
I have been looking at that small c on the Muncies in 68 GM vehicles for about 5-6 years. It seems to be an inspection mark, but not 100 percent sure on that.
As already mentioned the 3915085 transmission tag is correct for an M21 in 1968 used with 3.55 and lower (but numerically higher) rear axles. So it is correctly matched with your BU rear.
G1 on your rotors is Detroit Gear and Axle plant, 1st shift.
Some white lithium grease on the shifter is not a problem.
Try to post a picture of the inside of the deck lid in the area right under where that tape is. I am pretty sure I know why the tape is there. Probably because the emblem was relocated higher up on the deck lid (second set of holes made) so the rear spoiler could be mounted. Tape kinda helps cover those holes.
I will try to look over all of the pics. Just quickly went over them for now.
I think I would put on a new fuel pump (but keep the old one). I agree it may be putting fuel into the crankcase.
As for oil type and brand, lots of debates on that. You might consider one of the oils for diesel engines. Use a good Wix or Napa filter.