james, love the look of brushed aluminum.
joey, the joining point of the obd1 m exhaust is fairly easy to weld to. going all the way up to a 3" diameter would be easier to do with an adapter from 2 1/2 to 3".
i used 2 1/2" pipe on cadence, and then simply ran it back all the way @ that diameter. i just ovaled out the one end a tiny bit to get a tight fit and used the arc welder.
till the muffler fell off....
(stooopid trans mounts
of course the fact that i may have been standing on it a bit and twisting the bejeebers out of her on every take off out of the toll booths....or that i had only 1 support on the muffler....naaah....don't meen nuthin'....
since the front pipes are already cut, though, and is where you are having a fitment issue, i'm not convinced that your midpipe would be the best solution. i have another m50 front section, from a 525 e34that may work, though. i'll have to measure the pipe diameter, and can compare it to the //M exhaust i had on cadence for contour.
if it looks like it would work, i could even weld on a 2 1/2" to 3" adapter to the rear of it for your cat location.
it's dark now, but i will get the pics asap for you, k??