...and if you DO seam-weld, grind the goopy undercoating off where U are going to weld at the very least.
that way 2 things happen:
1--welds are better quality and U don't have to breathe as much toxic smoke
2--U won't catch the goop on fire and damage other important stuff on back side of the panel you're welding. Don't ask how this was learnt......
Be expecially careful when seam welding in the footwell areas, lots of important wiring you don't want to get cooked.
Long-term, it's best to remove all the body putty. even if you do prep the actual weld arreas and leave rest of the goop on, the welds will lift the coating a bit and let water/salt/road grunge in, and this leads to localized rust probs after a few seasons.
and as to the question of seam weld or not, our team's second suby shell was not seam welded. after a couple of events, we could see how panels around the transmission tunnel had pulled apart 3/8" in places. Car got seam welded shortly thereafter. MUCH easier to do when car is totally stripped for cage work rather than after it's built.