Instead of welding them to the adaptor, why not index them to it using locating pins. Use longer bolts and custom hold downs, then let the locating pins hold the manifolds in place. The problem I see with the whole welding process is heat expansion. That much cast iron is going to move around after welding from heat of the exhaust and something will give way.
Maybe you could bolt the manifolds to the adaptor from the back side with recessed bolts, them bolt the whole assembly to the head with factory mounts. Or, see about a oversized water jet cut flange so you can bolt around the edges.
I just don't see weld as a very good option when then are other ways of getting there.
