[ChevLoRay]

Somebody correct my thinking, if I'm wrong, but you have to think that it won't move anywhere near enough to let the slip yoke come out of the tailshaft, or the front drive shaft. With that said, you can look at the slip yoke and see how much of it is shiny, and that should tell you how much movement there is in the driveline, which should include some minor movements of the engine mounts and the transmission...unless they're on solid mountings.

Since you have leaf springs, I am of the opinion that movement would be minimized, compared to the trailing arm set up that I have on my truck. If you're concerned with the punkin' hitting the gas tank, see about locating one of the thick plastic shields that FoMoCo installed on early Pinto's. It was designed to prevent the gas tank from being punctured by the rear axle, in a rear end collision. Didn't help the Firestone 500 tire issue, though.

I don't think you'll find much movement to worry with, as long as you have a couple inches between the tank and anything that moves in normal suspension travel.

Again, this is my opinion, based on my (ample) gut feelings and after having worked under more than a couple cars on racks. It may be totally wrong, but I feel good about what I have said, until someone with more experience proves me wrong. Tread lightly.