Well, we moved the plane.
I wanted to write this up for posterity if anyone else attempts something like this.
The first thing I noticed was that it has a consistent stumble when opening the throttle. Possibly a dried up accelerator pump diaphragm? It recovers quickly and it's smooth when operated slowly, I just did that. Several full power static runups and aborted takeoff runs were completed with no signs of power loss or roughness, so a go decision was made. Even with inspections and tests, taking off above the alligator infested lake near the airport was one of my braver moments.
We got a nice backfire (afterfire) upon pulling back the throttle at top of climb. Interestingly, this did not repeat itself for the rest of the trip, but research pulled up discussion of the "Grumman Stumble". I do have the 10-5217 carb with pepperbox main. Maybe it's plausible that some of the holes were clogged with old avgas. Other than this, performance was generally what I expected, with climb rates >500fpm and cruise around 110 KIAS.
I suspect the FS450 reads way high, as it indicated around 12gph leaned in cruise at 4500 and 10gph at 8500. This, with the EGTs around 1500-1550. Closing the throttle at all would cause EGTs on cylinders 1 and 2 to drop to 1300 and cylinders 3 and 4 to climb toward 1600, so I tried to avoid that. It would still run smoothly, though, and CHTs were never a problem, staying around 400 in initial climb and dropping to 360-380 in cruise.
At one point climbing out of 8000, we got a major stumble and a series of bangs out of the exhaust. I had the fuel pump on, switched tanks, and mixture in before the other part of my brain asked what was going on. I decided that I was above an airport (yay for planning) and it'd be foolish to try to jump to the next one. This would turn out to be a financial mistake, but a good move nonetheless. They found 4 spark plugs reading in the 300k—1MΩ range (two of them were on cylinder one). The gaps were also all too wide. I have a log book entry for cleaning, gapping, and testing the plugs from 5 hours previous, but maybe given their age I feel hesitant to start pointing fingers.
We replaced 4 plugs with overnighted new ones, and I removed the carb fuel inlet screen again to clean it out, but there wasn't much in there. It ran noticeably smoother after this, but the partial throttle EGT disparity between 1+2 and 3+4 remained. It developed an oil leak behind the newly replaced vacuum pump, so it's hard to nail down numbers, but we consumed 1 quart the first hour decreasing to 1 quart over the last four hours.
It's now stored safely while we wait for time on the schedule. I ran out of time away from work so I've asked them to cut the filter so we can make some choices. After talking with the shop owner, I'm leaning toward an IRAN (gaskets & cam) and top with high compression pistons. At least if we pull a cylinder we can make a call on the cam and splitting the case.
So that's it. The theories that O-320s
generally seem to fail over the long term through loss of performance rather than in spectacular fashion seem sound. On an old engine, I'm way more concerned about fuel issues than metal breaking.
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