Winter is a good time for projects like this. No amount of large pipe wrenches, pipe extensions, PB blaster, heat, time, would budge this 2 1/2" bushing from the manifold flange. No worries - time is on your side in winter.
I started consulting plumber friends and researching household plumbing. That field deals with frozen bushings all the time.
A bushing in an exhaust flange that's been together since 1974, pales in comparison to a bushing in a 100 year old, steam radiator.
With a Sawzall and metal blade(after cutting the 1 1/2" pipe flush to the bushing), I cut across the top of the wrench side of the bushing, stopping at the exhaust flange.
Then with the piece in a vise, I turned the blade into the inside of the bushing, and slowly/carefully, cut -almost- into the threads. One cut and into the wrenches, the outer bushing began to collapse but didn't budge.
Next(back in the vise), I made the same cut 180 degrees away. With one wrench and 4' pipe on the ground, and the other with 4' pipe 45 degrees up-with all my weight- I felt it budge. A little more penetrating oil - all my weight again and again, it finally turned out.
This took about an hour - an energetic hour.
Just one tiny nick on the outside of the flange, the threads are untouched. The flange appears to be a thick bronze casting.
Now I can start collecting piping for the riser. The old exhaust was 1 1/2" in the dry area and then increased to 2" after the water mixing muffler, with no turns at all.
With my engine(4 cylinder 30+hp), I wanted to increase the diameter as the new design has elbows and of course water in the hose, after the muffler. The riser will be 2"(after a 2 1/2"x2" bushing), black iron pipe and the hose -to and out of the muffler- will be 2 3/8".
The muffler is a larger size for added capacity(1 1/3 gallon). That and the increased hose will give a larger volume for containing the water left in the system at shut down and for prolonged starting(which I don't forsee but somebody else could do it).