Update:
We ended up doing it your way after all.
buddy came over with a MIG welder. We took off the muffler, and I had a go at sawing the slots in it, but with a hacksaw it was going to take quite a while.
Pal shored up rusting support hangers with extra welding, while I gave up on sawing the slots. He suggested (as you did) to just weld it.
With the back wheel off there was plenty of room to weld all the way round (a concern of his).
We tried to remove the 2nd (old, std) muffler on the driver's side. That muffler has an extra 3 ft piece of pipe attached, and also a kind of built in heat guard (I think its the $160 muffler, the other one was $105 last time).
Anyway we got the U-clamp off, but couldn't go any further with it. We're starting again tomorrow.
On the other hand, now I have two working mufflers: the old (still working sort of) muffler on drivers side, and the new "Flowmaster"-style Stainless steel musclecar muffler on the passenger side.
The car now has a deep but smooth rumble sound with normal driving and gets a bit louder (something like a noisey muffler) when accelerating. It aint too bad.
I am wondering what the car might sound like if we get the other (virtual cherrybomb/flowmaster) muffler on tomorrow.
Having a friend with a welder and some time really makes a difference in getting a job going.
He said to cut slots I should use a 'sawsall'. Of course the most advanced tool I have is a crappy jigsaw.