I took the frame to two different shops. The first shop specialized in race suspensions and had experience with Independent Rears. His focus wasn't so much the trailing arms as much as making sure the axle center lines were the same and the pinion angle was good. This shop had the frame for a year (long story) and placed the cross member in place and verified pinion angle was a positive 3 degrees. This is important for two reasons. The first is the most obvious, the drive shaft angle. The second was so the lower control arms are parallel to the ground, this allows the bearings to wear evenly. One the upside, he didn't charge me for welding in the cross member or the other work/measurements he did. And he was closing shop because he didn't have time to keep this work as his primary focus.
The second shop I took the frame too kept it three weeks. Last year, he put a 'vette IRS into a '61 or '62 Bel Air. He and I had some good discussions around the trailing arms and the shocks. In the end, he said the Jag was easier and didn't require as many modifications as the 'vette rear.
Because I like to be engaged with any shop I use, I did a lot of research on what works best and avoiding bind in the suspension. I did a lot of work on this site:
http://irsforum.boardhost.com/index.php
This research really came in handy and both shops liked my input/thoughts.
With the trailing arms, length isn't too big of a deal. The mounting points however, need to be done in a manner which doesn't bind, otherwise, a lot of wear will be caused on either the lower control arm bearings/bushings or the trailing arm mount points. Stock Jags didn't have this problem since they used rubber bushings on both ends of their trailing arm. This absorbed the bind, especially in non-high hp applications. In a lot of cases, trailing arms aren't needed. In my case, the are needed, badly. They will cut down on the wear on the lower control arms during acceleration. On a hard acceleration with lots of tire grip, the hubs could break. The trailing arms help strengthen the lower control arms by not allowing them to move perpendicular to the shock travel.. To prevent bind, the trailing arms
must pivot on the same plane as the lower control arms. Otherwise, they push/pull the lower control arms as the suspension cycles.
The direction of the trailing arms is more of a clearance and aesthetic concern. Since I didn't have clearance to run them to the front (the frame got in the way), I decided to run them to the rear.
I will be running a total of four shocks, just like the stock Jag would have. Even though the El Camino isn't too heavy, the work required on the shocks is a lot.
Let me know if this answers your questions or sparks new ones. I'm happy to share what I've learned.