Muffler exploded; LITERALLY!!

Probably unburned fuel accumulating in the exhaust and one pop.

Is the engine extremely out of time?
I've heard if the timing is way way off that it can cause a back fire big enough to blow an old muffler apart.

Definitely unburned fuel.

Now you need to find the cause.
Start by checking the firing order and your plug wires.
Next, determine TDC.
also, check for any loose or unplugged wires near your distributor.
Pull the plugs to see if they are wet.

PS the muffler is NOT a combustion chamber.

I saw a guy do that once who had installed an aftermarket ECU on his Mustang with a flowdisaster exhaust system. He decided to use "spark cut" instead of "fuel cut" for his rev limiter lol. The moral of the story is that retarded timing is the most likely culprit.

You might be a little rich, but until your up to temp and off choke you can't really tell. Set the floats according to the FSM and tune with the adjusting knobs, not the float level. You probably just flooded it trying to start and unburnt fuel ran down the exhaust system, then when the spark plug fired on an open exhaust valve... BOOM LOL.

I run glasspacks so I don't have those problems