I'm amazed. all the know how on this site and nobody sees the tools they have around them?
ok you need to weld the manifold while hot. that helps deal out the prospects of cracking after the weld is laid. use a freaking Barbecue! put some foil down if you want but its a perfect annealing chamber. set up your work and then crank up the gas grill and bake to about 400*f. once hot open the lid weld the thing and then close to bake in the heat. let cool after you have finished both sides. piece of cake!
oh yeah doing like this you can get away with a mig welder flux core or not.
if you have a stick machine use the nickle drag rod. that works great.