Popping out the exhaust, rough idle

Hi,
My GMC Sierra C3500 with a 454 engine (non-dually) is popping out the exhaust when I drive slowly, especially when I let off the gas and start to slow down, running terribly, low on power, sounds bad too.
Also started throwing po174 code (but this happened soon after I clean the mass airflow sensor, so I'm not sure if I triggered it or if it's the truck triggering it).
These symptoms occurred suddenly a couple weeks ago after a weird thing happened : was cruising at like 65 mph, truck running just fine. When I noticed smoke rising up in the rear. I pull over and there is a smoldering board laying on the exhaust between the cab and bed, and then there's something stuck to my exhaust system underneath the truck on fire! I'm assuming what was stuck to the exhaust system was some kind of trash or something? A police officer nearby ran over and hit it with a fire extinguisher.
So I get back in the truck, crank it up and take off and the symptoms start. I don't know if the fire was a weird coincidence or what?

One shop said I have a burnt valve and a mystery vacuum leak, another local shop said that they are "90% sure" it has a " burnt valves", but after reading your article about back-firing, I am not so sure.
Second shop claimed they did a pressure test, but I don't believe them.
I don't like their "90 %" odds because the burnt valve repair is going to cost me like $1,500.00 - $2,000.00.
However, they might be telling the truth because I did the dollar bill trick in front of the exhaust pipe and the dollar bill definitely flaps in and out.

I have changed the plugs and wires, got a new muffler, cleaned MAF sensor, new kn filter, and deleted the rear catalytic converter because one mechanic said that it might be clogged (previous owner deleted the rear O2 sensors, so it didn't really bother anything).
What to do?
Thank you!