You can use clecos to hold panels together when doing butt welds.
http://67-72chevytrucks.com/vboard/s...utt+weld+cleco
The tighter the gap the less warpage you will introduce into the panel, the ideal gap is no gap at all. Those so-called butt weld clamps that Eastwoods sells are a complete joke IMO.
Granted vise grips work great but the good ones with enough throat and depth cost about 40 bucks a piece. Even then the longest ones I've seen are about 30 inches long...Eric