do a compression check.
here is a sad story for you about old gas and a 1973 impala with a 396.
Recently helped a friend get his car going again. had sat for 3 or 4 years with a bad driveshaft.
I got it to run fairly easily thanks to a self draining q-jet which didn't gum up with old gas.
I got the engine running with a 5 gallon tank and a small electric pump.
I thought the tank was empty because the gauge showed empty and the mechanical fuel pump wasn't pumping out any fuel when running (using the external tank). turns out the gauge is broke and the rubber line from the tank was mushy and flat (not flowing).
so when I added 10 gallons of new fuel, the gauge still read empty and the stock fuel pump wouldn't pump (until I changed the rubber feed line)
anyway, got it running with the old gas and new gas mixture. The exhaust smelled like old gas but the car was running fine. Took few days of tuning the car and getting it to run perfect. Then it started to miss a little and it got worst and worst over a few days. Finally the engine would barely run and had no compression on 4 cylinders.
What happen? Well the old gas gummed up the intake exhaust so badly that they stuck in the guides. This caused the push rods to bend and the cam/lifters to started going flat.
I pulled the heads and had to hammer out the intake valves. There was some sort of varnish on the valve and inside the guides. Lacquer thinner was the only solvent that would get the varnish off.
After taking the heads apart and cleaning them, I re-assembled them and changed the head gasket, cam, lifters, and straighten out the pushrods.
A terrible experience which I know was caused by running old gas, even through is was mixed with new gas.