I'll take the time to describe the past later, this is the big project I finished today.
Started with this collection of parts:

I abandoned the idea of using the 20* fittings and went with 2" to 1-3/4" adapters instead.
I did a lot of grinding and fitting with the angles and could not come up with a way to keep the angles set from mock-up and fitting to the transport to my welder's. I like it straight better.
If you want the down and dirty, I simply ground down the old muffler to manifold welds, leaving the top weld seam as a keyway of sorts. Then, I pounded those 2" adapter onto the manifolds and set the angles with the mufflers slipped on. Carefully slipped them back off and went to get them welded. Worked like butter.