Jim,
I too had serious concerns about drive shaft angle when I was completing my car after a tranny swap. Due to the very short drive shaft that I had to deal with, the 1/2 inch horizontal offset I had between tranny and pinion resulted in an angle of 7 degrees. The pinion and tranny are as close as I could get to being directly inline vertically. The rear is IRS like yours, and bolted to the frame, so I don't believe there is any roll to experience either under power or brakeing. Tom Wood, a very knowledgable guy assisted me with this set up and provided a drive shaft to span the short distance. I have noticed no vibration due to the angle, and after three years and about 8000 miles have noticed no abnormal wear to the U joints. In fact I just pulled them last week to inspect them again.
There are standards that we all try to achieve, but sometimes you just have to deal with what's on your plate. Go with your best plan, and evaluate your results. If you have to go back and change things, consider the whole thing an education
Good luck and don't loose too much sleep on tring to achieve the perfect angle.
- Jim -