MIG is fine mate, the main pain is getting all the underseal off (both sides) prior to welding, I'd have thought most mobile welders are either MIG or Arc.
Most mobile welders should be able to do it, it's a couple of hours work if you've prepped the whole bay, also worth getting a bar welded inbetween the NS chassis leg whilst the welder is round.
Read my thread for the process and post welding prep (takes a while)