Super six set up with low comp

Are all the cylinders at 90 psi? If so, that is a bit unusual for all 6 cylinders to wear out that evenly, and for it to run well with that low a compression all across the board. That would make me question the compression tester and how it is being sealed in the spark plug holes. It could be for real, but I would be looking at some other cause for those readings.

And yes, the 1980 motors were 'hotter' than the early motors so the smaller difference that you found is rational. The /6 cams were changed in the early 70's for example.

For the OP, I would suggest a re-test of the compression, with all plugs out, cranking 5 times per hole, and with the carb propped open, and see if the readings change. If they are that low, then put a teaspoon full of oil in each hole and retest to see if the oil increases the compression readings; if so, then indeed you have worn comproession rings.

Putting on the super six with worn cylinders won't hurt anything, and ought to perk it up a bit, but of course it will still be down on power due to low compression. I would not expect any big mileage change with the exhaust already changed.