Good point on the driveshaft thing. Maybe related. And here's my counter-point...when I replaced my FGF open rear end with a posi rear-end out of a 74 Nova, I also had to shorten my driveshaft and change a yoke out. My driveshaft shop noticed right away that my original was out-of-phase, and when I carried it into the shop the owner said "must be out of a first gen camaro/Fbird?" so he recognized it right away as being unique to those models, without me telling them first what it came from...also ID was easy he said based on the oddball diameter. Then they told me they ALWAYS rebuild these shafts in-phase because they could never find a compelling reason why GM did this, and they've never had one come back for vibrations. So I took their word and they shortened mine and re-yoke'd it in-phase. And I also have no vibrations at any speed.
Of course countless other driveshaft shops sell shafts for these vehicles. With the exception of Dennys, the vast majority seem to line the u-joints up and sell them in-phase.
Frankly it seems to me that equal success is reported with vibration control, regardless of method of driveshaft configuration.
