I have heard that same suggestion from Drive Shaft shops about slipping the yoke in until it bottoms, then pulling it out an inch, but.....
MOST factory built cars seem to have more than one inch sticking out. Most of my junkyard experience is with A bodies since they made so many more of them. Most had between 1 1/2" to 2" sticking out. Now, this is with the rear axle at full droop. Still, looking at it in terms of geometry, As the axle droops, it also swings forward because the leaf springs are hinged at the front eye and shackled at the rear. Downward travel causes the yoke to slip out but the arc of the axle as it droops minimizes how much. I have not closely studied this but I would suppose that even at full droop, the yoke probably does not move more than one inch. This may explain why the driveshaft shops use the one inch guideline. They surely do not want to have the drive shaft bottom out.
I currently have a 65 Dart that I'm working on. It too has about 3 1/2" of yoke exposed with the tires at full droop. This car was originally a slant six/904 but when I got it, there was a 318 and a later slip yoke 904 in it. I have no way of knowing what this driveshaft was in originally but the car drives fine with this setup. If I were to have bought this car and it had no drive shaft, I would defer to the common suggestion for measurement.
Final note:
In 1984, I had a friend that was given a 74 4 door Valiant. He really had no use for the car so he decided to have some off road fun with it. He cranked up the torsion bars and did something to lift the rear end as well. Driving down the road, the car had an annoying hiss type sound, almost like what you hear when brake drums are on a lathe....This went on for a couple of days. Something fell out of the rear suspension and the rear ride height was back to normal. That hiss sound was gone too. We thought back then that maybe the sound was from either driveline angle or from the shaft not being in the transmission deep enough.