On the 1500, there are two square head plugs, one on the bottom, and one on passenger side near the bell housing. There is a hex head bolt just aft of the filler plug that holds part of the reverse mechanism in place. I filled mine before installing, but I did spill a little out the back during install. Luckily, I anticipated this and overfilled just a tad. The worst part of the job was washing the gear oil out of my hair: took 4 tries!
I would pull that bottom plug (over a pan) just to make extra sure there's nothing in there. Also, considering the trouble you had with the shop, I would check everything over real well before you install. Put it in all gears and spin it in each gear. Pull the cover on the the top and make sure there are no rags or loose bits in the case.
Just my $0.02.