There is available a "step down pipe stub" to overcome 2 identical diameter pipes not joining.
Hey bullet man. What I did was found a peace of pipe that fit snug into my muffler, but welded the pipe onto the header and clamped the muffler /adapter. Worked a treat and meant only 1 clamp. There is an exposed weld, but I'm going to heat wrap the header and cover that.
Goggle it, & search on Ebay. Also try the big M.C. parts houses "Bike Bandit, Bruce whatshisname". You Will find one if you look hard enough.