Alright Chris, here’s some thoughts I’ve got going on.Glad to see you're still at it Dave! Not sure why I didn't see you tag me on Sunday, but anyways...
I would stick to 1-1/2" stock if I were you. I prefer tubing (1-1/2" DOM .120 in this case) but I think you would be fine with square tubing as well. The reason I say nothing more than 1-1/2" stock is once you go over that, you risk having some interference issues with the upper control arm mounts on the IRS cradle depending on where you locate the lateral support.
In the pics below, I used 1-3/4 .120 DOM tubing for the lateral support between the frame rails. I have the tube pressed tight against rear floor plan and with the IRS installed, I have less than a 1/4" clearance between the upper control arm mounts and the bottom of the tube. With tubing, you can play with the positioning a little to gain more clearance, I just didn't think that far ahead and got really lucky on mine . I didn't do any bracing like your planning to the frame rails, but I did weld the lateral support to 1/8 plates and added two triangulated gussets from the tube to the frame rail on each side to provide additional support and provide clearance around the upper control arm mount.
First, i need to maintain the backing plate for the rear seat belts. I will remove the entire upper control arm bracket as a unit, then i will either need to trim off the plate that backs the seat belts, or possibly make a new one that I’ll want to weld to the cross beam.
Though a straight beam going from frame rail to frame rail is ideal for strength, i’m wondering if some sort of multi-angled support that allows me to get towards the curve of the rear floor pan could work? I’ll want to weld the seat belt reinforcement to it for the added strength.
Maybe I could think of a way that this under floor support could weld into the new x-brace? This would allow lateral forces to be directed straight to the subframe?