Great! Thanks. So carnewal cuts a window in the top and welds it back up. Doesn't leave a lot of room for welding inside. I assume they do a different mod. I think the seam split is still my goal.
I need a metre of perf pipe and two times
MUFF PACK
I'll fill the remaining cavity with loose fill fibreglass packing.
Result should be quiet, nice sounding, and free flowing without drone.
I've been looking at Magnaflows but the volume is too small or the internal piping is too big.