Help, my exhaust is killing me.

Update: I did not have time to pull any plugs, but it gets interesting anyway.
A friend of mine took a look at it this morning, he was the one who wanted to advance the timing.

On the way to his shop, I put a few gallons in, (no gas gauge), but this time I selected the highest octane. I know I have before, but not regularly.
He checked the timing but made no changes, and he adjusted the carb.

And no more sickening smell!

I am still in disbelief. I even made him drive it for 20 minutes, and took it for a ride myself for another 20.

Was it his adjustment on the carb? Possibly. It was already lean, but he did some adjusting and set the idle a little higher. He suggested I drive it like that for a few days and then he would set it again, at a lower idle. He also agreed that I should replace the 750 with a 650.

Was it the higher octane? Probably?

We put it on the lift and I noticed one of the mufflers must have experienced a backfire at some point. It was not leaking, but the baffles inside are probably messed up, which may not help.

Also, could it be possible that a rich mixture combined with bad manifold to pipe gaskets affected the mufflers and exhaust, maybe filling them up?
It must be the first time in a while that the all of the exhaust gasses are actually making it all the way through.

I'm still not convinced it is solved, its difficult to really tell right now, I need to leave it be for a few days and see if I smell it again.