Are you planning to weld mild steel straight to the cast manifolds? If so, I think you should build some proper headers/manifolds from mild steel. I can give you the contact info of a dude up in Canada (Burlington, Ontario) who CNC cut the flanges for my Explorer. In fact, he probably has some flanges in stock for the OHV engine. I'm 100% sure he has the OHV's pattern on file so if he doesnt have any OHV flanges in stock, he can probably cut a pair in no time.
This is a pic of the SOHC's flange for my headers: