I think I know what you are asking, I had a GP Giles with a .68 mds (dog engine) but I had to go to the Macs Xlong header to get the rpms I wanted. The Cut was kinda on the radius of the header but there was enough left to fasten the mcm to.
It worked great gained 1200 rpms over the stock muffler. I have been fooling around with these muffler quite a bit and have found for the smaller engines like the 46fx that a ronsen butane lighter(smaller can) refill can and a 3/16 stinger with about 3/4-1 inch sticking out will add at least 1000 rpms over the stock OS muffler.
I have had a few engines such as my Webra's gain 2000 rpms Try running a 11-4 APC prop.
If you don't like the OS Try a Webra .50 speed more power swing a bigger prop 11.5-4 apc.
Scott