That muffler was also recommended to me by a very savvy 4x4 shop owning friend and I've now used it on a couple of trucks with great results.
MISF came up with a leak-proof 3 bolt flange system that I've used a couple of times. They're a bit laborious to make and while not needed, access to a lathe makes part of the process go faster. If CAD is in your wheelhouse then Oshcut or sendcutsend can make the flanges with the two different ID's and save you some time. That is what I'll be doing going forwards.
It goes together like this:
The up-stream flange (on the left) fits the exhaust tube exactly and is placed ~1/2" up from the end of the tube. The down-stream flange has it's ID opened up to fit over a piece of exhaust tube that has been expanded to be a slip fit over the tube size chosen, and is welded at the end of the tube. No gasket required, a little high temp RTV in the 'corner' of the upstream flange and tube is all that it takes to seal the joint. Using these joints makes it possible to remove sections of the exhaust as needed to work on other parts of the vehicle. Another trick with these joints is to use Grade 5 bolts, but use
Brass nuts with only flat washers. I never, ever use split lock washers on anything and I have
never had one of these joints leak.
This is the new up-stream flange in place under my FSB:
And this is the old one, from before I replaced all of the system with the correct emissions legal parts:
This the system for my old '65 Valiant sun curing the bbq paint before going under the car for good:
As can be seen, I also use the flange bolts to also attach the hangers
With the now easy availability of the Marmon Flanges (commonly known as "V-Band" flanges) some might wonder why go to all of this trouble? I've used the Marmon flanges on a couple projects and I like them, but they do have one feature that is both a blessing and a problem. They have no fixed rotational orientation. Great for twisting something to align it, bad when something wants to twist out of alignment. On one application of Marmon flanges I 'keyed' them so that they could only go together in one orientation, but that needed a mill to make happen easily and it compromised the sealing of the flanges (in that case it was only cold air and we could stand the leakage). I wouldn't do that with exhaust.