The 1968 YG5-T that handed my old Suzuki TS90's it's ass in a flat track race had an expansion chamber that looked nothing like your fabricated one. I'm just saying that the old GYT kit chambers were smaller and much more well mandreled. If anything they were very minimalist in bend but had a generous sized chamber and a shorter stinger.
I'd be focusing on upping the compression some and getting into the ports and smoothing the heck out of those old rough-cast transfer ports and passageways. The GYT kits also came with a really radical looking, huge radially finned head, I hear they're quite rare now.
We're here because of a love most exestential. (toothy)