Bumper shocks

I have a '75 280, and I'm trying to compress the rear bumper shocks, so I can get a 240 bumper on. Now, I've already loosened that screw to let the air out, but the left one still has about 2 inches left where it seems it should be able to compress more, but it wont budge any farther, after relentless beating. The right side wont go in at all, any ideas? I really dont want to have to drop out the tank and remove em. :stupid: