You won't need to box the entire frame, just from about the front motor mounts to the trans mount. Move the plates into the frame channel about 1/2", leaving a lip on the top and bottom. Use a block of wood at each end of the boxing plate (between the plate and the back side of the frame channel) when you are positioning it in the rail. Push the plate up against the two blocks of wood to keep the plate positioned properly while getting your tack welds in place. That way, you have a recessed channel to mount fuel lines, wiring, etc. Don't weld any more than about an inch or two at any one location. Weld a little at one spot and then move to another location and weld some. This will keep the heat of welding from warping the frame rails. Just keep stitching it together until you have completely welded it.