I am sure your welder knows what he is doing, maybe you could ask him what he is doing so you can post the procedure for others to use? That mount looks great, should be rugged as can be.
My fire truck project may not be doable, at least not like I wanted to, I would essentially need to cut the original frame off behind the cab and set it on top of an entirely new frame, this would give me enough clearance with the engine and transmission, but then it raises hell with my rear axle height. To top it off, I am trying to mount a knuckle boom behind the cab and they want the frame to be no higher than 42”, that isn’t going to happen with the larger tires.