hmm just come across this thread and its exactly what i'm doing right now with my R31 skyline shell. TIG welding ALL (yes all) the seams and joins of the body. Though i'm only welding in the cabin area, as front and rear has strut braces going on. TIG is better than mig as it moulds the joins together rather than adding to them.
The only downside to welding most seams in your car, is you go through a lot of gas, and gas refills are around the $90-110 mark.
And yes, it is really easy to blow huge holes in the body if ya not careful.....practice on something first !!
I'll have to take pictures and post up my R31 body.