Slant 6 won’t start after 4 months

Check the timing. You can check timing "on the starter." Also HOW DID (exactly) you check the spark?

EG: If you jumper the start relay under the hood to check spark, THAT DOES NOT give you a clear picture, because the ignition switch circuit CHANGES from run to start.

You need to "rig" a WIRE core wire, even a low voltage piece hung "in the air" and to a spar gap. Crank it USING THE KEY. You should get a nice blue "snappy" spark at least 3/8" and typically 1/2" long

Clean or replace the plugs and RUN A leakdown/ compression check. It may have DRY CYLINDER walls and low compression, and running gas in there did not help., Try squirting a few shots of oil in each cylinder.