I just put one in a 71 with fresh paint and the tank in the truck

Not a nice job! I have also gone to just GM, but the 67-70 style from GM appears to be the same repro we have.
Anyway, I used mild liquid soap, diluted with water and pressed it in using wooden paint sticks...no screwdrivers or sharp metal tools. It does NOT like to go in when the tank is installed!