My First Mopar- 1966 Barracuda project

BenNSB said

Less angle on the shaft. Shortest distance is a straight line. If the rear sags, the shaft pulls out a bit. Might be closer to a 1/4” but for my situation it’s to the good.

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When the suspension sags, the rear end actually moves toward the transmission very slightly because the rearend/spring is pivoting on the front mount and travels in an arc. It's a very tiny amount though. On my 66, it amounts to about 1/8" - completely insignificant.

If you're simply move the front mount upward, I doubt there'd be any measurable difference in distance between rearend and transmission. Either way, with 4" of spline engagement, it's a very moot point.