I definitely am along the lines of more is better, because I want to keep at least half of threads if not more of the rod end threaded into the bar. The diameter of the rod end shank is smaller that the tube that it is fitted into. If it is necessary to maintain the 115" wheelbase, you can buy the 1 1/4 Chrome Moly tubing and buy the threaded bungs (left and right hand threads) and have them custom fitted and welded to your length. Sometimes the wheelbase is lengthened from factory specs to help stability on the top end and sometimes it is shortened to help on chassis reaction. How do the rear tires fit in the wheel wells? Are they centered or are they too far back now? I would rather have the wheelbase set at 114" than not have enough threads for the rod ends.
You spoke of the arch for the rear frame rails. Is there clearance problems with the rear end, or anything hitting the frame? If properly braced the rear frame rails are only holding up the rear body and a spot to mount the shocks. And it needs to have good welds and be square in the car. The rest is really cosmetic in my opinion, because your suspension is being held by the front crossmember. By properly braced, I mean 2 bars going from the top of the hoop to the frame rails in the trunk area. And where the shock bar and shocks are mounted on the frame have a X-brace tying back into the main hoop. There are different variations on this, but this is the basics in triangulation. For the front crossmember of the 4-link, I like to add some extra bracing at the top of the 4-links plates rear frame rails to the bottom of the hoop and also some going forward from the 4-link plates at a diagonal over to your frame rails on the underneath side. This will try to keep the crossmember from bending in the middle and the top 4-link mounts from flexing.
There should be some other good inputs on this.
Bill M