Just an opinion, but relatively few people have cured a fuel delivery problem in big glow engines by blocking or pinching a muffler tube. Some have, but most have ended up still having a problem that drives them nuts trying to fix it.

More have found the cure in relocating the tank to have a shorter distance from the tank to the carb, and to place the tank center line closer to even with the carbs fuel inlet. Others have cured the problem by adding a pump or regulator, and/or a check valve in the muffler pressure line. The reason for the check valve is that low rpm throttle settings will let the tank pressure back flow through the muffler, reducing tank pressure, and making idle and low throttle positions nearly impossible to maintain.

Many of the large displacement glow engines suffer the problem you are experiencing, so don't feel alone. Start by adding a one way check valve to the muffler pressure line and see if that provides a cure. It's the cheapest of the bunch and may be all you need. Butchering the muffler can end up a costly method of attempting a fix, and often doesn't work.

You will pick up a few rpm (100-200) by cutting the ends of the pipes if they currently have a reduced (pinched) diameter at the tips. This may also help in the fuel delivery issue by increasing the scavenging effect of the engine, causing a higher fuel draw. If they are a constant diameter from the beginning to end, then you won't gain or lose anything by cutting them.

Again, just an opinion. Good luck.