Not to be a stick in the mud but I've seam welded 2 cars, have my own rotisserie and everything. I'll never do a 3rd and the sum of money that would change my mind no one would be willing to pay.
The wire wheeling (the wires flying off the wheel and sticking in whatever part of the body they hit), the using a carpet knife to get what the wire wheel couldn't, the spot weld drilling.....then there is the welding itself, ospho, etch prime, epoxy prime, re-seal and paint.
Did I mention the wire wheeling? That seam sealant comes off like a hot cobweb
The fire wall seams you have to do aren't too bad though. Just be careful about the fire wall insulation behind them. If the seam is an overlap which most are, welding one side doesn't help much, what I do in those situations when I can't get to the both sides is use a spot weld bit, drill through one panel and fill that with weld. Basically additional spot welds since it's joining the panel in the middle of the over lap and not just on one side