Well, again, we don't really know the true temperature without sticking a candy/meat thermometer down the dipstick tube, but is this something new or has it done it in the past? If it's something new, then I would start looking at the fan to see if there is something like a paper towel in there (see Mike Pickett's post of last week) or a mouse nest in the fan or on top of the heads - a Bore Scope from Harbor Freight would help a lot here. Get one with a light.
I sounds like classic clogged jets. It backfired because you had been pumping the gas and loaded up the muffler with raw gas from the accelerator pumps which, after a while, got ignited and BLAM!
I like your first fix - rebuild the carbs with lots of blasts of carb cleaner), but find your fuel filter(s), too, and replace them with new ones. It/they may be clogged.
Don't think #2 is necessary if it hadn't been doing this last season, maybe move the fuel lines if it is obvious they're getting hot (but again, if it was OK last season, skip this step),
#3 Yes, check the valves (should do that at the beginning of each season, anyway)
and skip the #4 re-jetting to a larger size altogether - Again, if it was OK last season, why mess with jetting? Jetting doesn't change over time - They just sit there.
That's my 2¢.
You've got my cell from the NE Speedster list. Call if you need help.