boxing in the frame rails

[chevytroy]

I have a 51 1/2 ton im working on. The first task on the frame is to box it in. my question is how to weld it. Im going to MIG it with my lincoln 255c. Im capable of spray tranfer but it can only be done in the flat and horizontal position. Short circuit can be done in all positions but does it have enough penetration the frame rails and boxing plates i made are 1/8". If any one has any insight or experience it would be greatly appreciated. I cant find any info on it.