Well I went to the track this past weekend with my 1978 with the LQ4/4L80E, and my assumptions were correct. I need trac-bars BAD. The rear end rapped pretty bad and it wheel hopped from a hard dig. There was visible damage to the u-joint strap bolts, because the bolt heads made contact with the drive shaft at some point. When the truck was lowered they used aftermarket hangers and removed leafs and a wedge to rotate the rear for pinion angle (non-flip kit). So the leafs are pretty weak.
I rolled it off the line with the drag radials (325/50/15) at 14 lbs and the truck went best of: 12.87 @ 105 with a 2.00 sixty foot. I played with the truck off the line and went 12.99 @ 105 and 12.90 @ 103
If I can get the sixty foot down I'm thinking 12.50's
My plans are to build cal-trac "like" bars.
I need to order the solid spring eye bushings, I've already bought the heim's and got some scrap metal around the shop. So now I just need the time to build the damn bars.