Do heat exchangers have different mountings? Options

[HeloMech]

Guy at work has a '74 2.0. He bought stainless heat exchangers about 2 years ago and we put them on. In the process, one of his exhaust studs pulled out of the head. So he had the typical tstststststststststststst leak at the head. He pretty much just parked if for over a year. This weekend, we pulled them back off, I tapped the head for a 8/9 stud and loctite it in. We let that cure up while we remove the front bumper to add a top pad, fix the passenger door handle, fuel leak, etc.

Soooooooooo... new gaskets, new nuts, etc. Put the exchanger back on, everything is snugged up... start it... TSTTSTSTSTSTSTSTSTSTST... leaks. Loosen it up, tap it with weighted hammer to get it to seat up in there and resnug the nuts... with the muffler end loose, it doesn't move.. it's for all intensive purposes, SEATED... tightly. Snug up the muffler end.. start it tstststststststst...

What gives? Could he have some wrong exchangers? the depth of the opening from the mounting tabs different? mounting tabs and surfaces are all flush/square and flat.

I told him to flip the car on it's roof and we'll fill the head ports around the exchangers with JB weld and duct tape... those fix EVERYthing!!!