[classicauto]

Don't really need to worry about the welds, its just that the position of the merge you were suggesting would make much more likely that they'd break down - they'd be getting pulsed by (effectively) a blow torch while under pressure. A typical joint or butt weld wouldn't have that much heat at the weld itself just due to positioning. Thats the only reason I said that - the position of the weld.

As for merging the wastegate runners, a shallow angle would be best, fairly basic really. Imagine that when the wastegate opens, you want the flow to be as easy as possible from the engine to the valve seat. That said, my a-spec manifol merges its wastegate runners on the inside of the turbine entry, at roughly 80 degrees to the primaries. It works fine, and they've only utilized a couple of the "rules" for good response/boost control.

But space will be the largest restricition and will likely dictate alot of the design.