Do you need to (by the rules ) have a working throttle?
The difficult part of opening the steel barrel is getting it to rotate smoothly and seal adequitely after the "operation"...

If you don't need to show that you have a working throttle, why not just open that sucker up...
You could easily go up to .250 (1/4" ) which would help quite a bit with the "flowed" muffler.
In the past, I have used a "flame" or Christmas tree shaped grind stone to make the straight through barrel opening into more of a true venturi shape. Then de-burr it so it rotates smoothly in the carb body. Just don't get carried away.

Speaking of which, you can radius the inside of the muffler's tailpipe opening to allow the exh. to flow more smoothly out of it.
Pull the rear cone off the muffler and get out the Dremel and put a nice radius around the opening of the drilled out "tailpipe".