The bottom of the manifold when you cut it open for access is far enough away from the main body so you shouldn't have to worry about warping.
You can cut it open with an air saw, plasma cutter or cut off wheel, what ever you prefer or have available, then cut out the floor and open up the chamber, grind/smooth down the edges, clean up the ports and your done. Then you can weld it up, should be Tig welded. If you can't Tig or don't have the equipment any welding shop could do for probably less than $50.00 depending on how big you cut the access hole, it only needs to be about 4"x 5", maybe bigger if you don't have a plasma cutter..