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.
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.
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