Looking to modify 2 barrel air cleaner base to 4 barrel…little help?

It doesn’t need to be so complicated. Measure the diameter of the ID of where the air filter sits in the 4-barrel base (in the picture you posted) and cut the existing 2-barrel air cleaner to that, lay the piece into the existing air cleaner, mark and drill holes then rivet it with shallow 1/8” rivets. Paint and done. Sealing with thin closed cell foam, a thin bead of sealant what have you. Piece of cake. Welding is overkill and unnecessary