Too much heat, not controlable, will probably warp the receiver.
IMHO. gas welding is not an option for welding a flat for the reasons just mentioned. You may be able to arc weld with a jet stick, I don't have the talent for it however.
You really need a TIG, or, if you have moderate talent, a MIG may work. Again, I don't have the talent for it. You may want to consider a ACE blank. However, you will still have the lower rails to deal with.