slant six cranks but won't start

You won't get the bad gas out by changing fuel filters; can't be done. You HAVE to drain the tank. After it has set for about an hour, pour a capful (maybe 1-2 tablespoons) of fresh gas straight down the carb throat, and then see if it will start and run a for a few seconds. If it does, then your gas is bad and it all has to come out. It may have gummed up the carb already. Since the gas is very suspect, and it sounds like you havae spark, do this test first.