The simplest way to determine minimum shock length would be to actually measure it. Loop a piece of chain over the frame and under a floor jack, hook or bolt it together, and start cranking on the jack until something bottoms out- - - -suspension snubbers, etc. Then measure the distance between the upper and lower shock mounting points and add an inch or so "fudge factor" so there's no way for the shock to hit a mechanical limit. That's how we did it on the round track race cars where there were no factory specifications available. If your shop happens to have chain anchors built into the floor, or the suspension can be assembled without a spring in place, it's a lot simpler.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!