My only advice on mufflers sold today would be that if you can see straight through it will be pretty loud if that’s the only silencers you have.
I would position the mufflers first. Some sort of stand to hold them up helps here. I’d cut the pipe ends (nipples) off the front (offset) of the muffler. Shorten the nipple on the rear as much as you can and weld on a flange on the rear of the muffler (centre). This helps with tailpipe clearance. I also welded a piece of 1” flat bar on the back at the top of the muffler bent at 90 deg to go through the muffler hanger.. You can weld onto where the barrels are joined to the end caps, bent at 90deg to go through the muffler hanger. Do you have both left and right exhaust hangers just in front of the diff?
I don’t have any sort of X-pipe. (Old school) My pipes are completely separate. I cut the ends of the extractors off and welded flanges to them.
Sounds simple but you then fill in the gap between the extractors and the mufflers. You may need small bends at the front of the mufflers. Weld the pipes into the front of the mufflers. This will make it one piece between the extractors and the rear of the mufflers.
Hardest bit is to keep it symmetrical. The Original Torana/HQ muffler hangers with new rubbers don’t have much movement so you don’t need a great deal of clearance between the system and the floor. 10mm would be heaps.
Look at as many pics as possible of other Torana systems.
If I was doing mine again I would buy tailpipes already bent and cut/shut them to suit. The tailpipes are the worst bit. I have seen Torana tailpipes advertised on some exhaust parts websites. So many variables but with swaybars, 9” diffs, drop tanks etc.
I would only do 2 1/2” tailpipes . I have seen some guys with 3” pipes over the diff but it isn’t easy to do without knocking and rattling.
I’ve even seen the humps over the diff modified to fit larger tailpipes.