I'm a newbee but I cant resist this thread. Im an old hot rodder rom the 70's. Owned a 69 chevelle big block for many years., also a former CTS forum guy.
Three things come to mind about this misfire that could be checked if they have not already been discussed.
Spark plugs if not totally clean with cause a misfire, replace new with OEM only.
Spark plug wires can break down easily. Spray with WD 40 to prove them out. If they are old change them anyway.
A lean condition of any kind will cause a bog rather then a misfire.
The key statement I read was that is was OK until he gave it a lot of gas "jumped on it" then bang. My instincts are suggesting electrical.
Is this engine using any oil?? Hot spots from oil deposits on plugs, valves, and heads can cause a misfire
Spark timing is critical as well, check it, prove it out.
Just my too sense, dont yell at the elderly!! LOL
Best of luck,
Joe