Sounds like a lot of work on your part ... and pretty expensive, too.
When you buy exhaust components, they're made for 2.5" OD piping. That's just the standard. You can, however, weld the pipes up buttjoint style in order to make resonators, mufflers, etc work. But the inside tubing on some of those will still be less than 2.5".
Of course, some mufflers will have bigger or smaller piping on the inside ... if the same case is used for multiple applications.
If you want bigger than 2.5" OD, they do offer 2.75" exhaust ... but it's a specialty size, and relatively expensive as far as exhaust.
I purchased a 2.75" SS section for IM project ... and it was more expensive than 3".
Not sure about MAM's midpipe ... but if they're made from standard exhaust components, it's probably only 2.5" ID at the flanges.
Most flanges have lots of extra room around the ID of the piping. It allows for adjustment in some cases, but makes it more difficult to weld. Too much slop. A flared end of a pipe, however, fits snugger than the pipe itself; makes it easier to weld.
Then again, I haven't seen the midpipe in person.
