What kind of balancer are you running..? Is it possible your timing mark is off...?
It is likely you are not at 40* advance like you think you are, which sounds like way too much anyways...?
I just melted down a fresh new 355 which I [thought] I had 35*, but turns out the timing was wrong and it was more like 42*.
It ran like a cat's butt on fire, but by the time I caught the problem it was too late and burnt several pistons about six laps into the race.
We did a test in the pits one night to settle an argument, and tried four top of the line expensive timing guns from other teams, and we got four different readings...!
We finally settled on the brand new best gun as the most likely accurate reading...?
If you are on alky, and are absolutely sure the timing is dead on, bump your jets up at couple sizes to see if it cools down.
If you are on high quality race gas, and you are sure the jets are right on, I would highly suspect your timing is off...!
I was also told by an expert engine builder it could have been an intake vacuum leak, but I didn't find any evidence of such when I tore it down...?