On my old D35 I had to replace the driveshaft as it couldn't be balanced. I got a Tom Woods custom drive shaft with an extended GM front slip yoke and cut it off so I had 1" of slip yoke travel at rest. It worked fine for normal driving until I floored it going up a hill one day which rotated the pinion. The slip yoke slapped hard into the transmission tailpiece a couple of times until I backed off. No damage done, but I shaved about 3/8" more off the slip yoke and it never did it again. So I'd say, yes, I think you need more than 1" of travel, more like 1-3/8" - 1-1/2".