Two ideas
1. Harbor freight bits are a bit cheaper (~$7 ??) than the Eastwood or Mac bits - but they cut a circle around the spot weld. The parts can be separated but the spot needs to be ground off.
2. Classic Motorsports in their last issue mentioned simply grinding down the top layer of metal, then using a sharp body chisel to separate the panels. This may be easier with a pneumatic chisel.
I use #1, but will try #2 next time around... and looking at the HF web site, a pneumatic hammer with chisels is $20. Gotta love/hate cheap tools... I buy the cheapo and try it, if I like it, I upgrade to a nice IR tool ($$).