After some plans and parts study I confirmed that the J-channel trim detail on drawing 18 is for the FORWARD end, not the aft end, as labeled. A test fit also confirms. I used the two F-786B's to match mark the F-786C's as both intersect the F-706 bulkhead at the same angle. Worked great.
I then marked and cut the aft end of the left F-786B to an angle to match the F-711. The aft inboard corner had about 1/8" to 3/16" interference with the HS attach bar. I started to trim the corner, then thought to use that J-channel to match mark and cut the bottom J-channel, since the 711 and 712 bulkheads are parallel. It worked fine, but the bottom channel came out about 1/32” to 1/16” long, and it was just too HOT in the shop to work any longer.