Spot Welder Cutter/Bit Size

I've got my tub unbolted and about to go onto saw horses for some sheetmetal work and I searched for spot weld cutters and there are a lot of sizes out there. Anyone care to point the light over to my direction to help me buy the best size or sizes for the job at hand. Mostly all I'll be workin on is drivers side floor board and toolbox area and the riser panel up to the tub bed and including the hat channels.
The JEEP is a 1964 CJ5, I picked up here last weekend from a fellow board member.
ShowMeGuy :v6:

 

Just got done with my tub. started with a cutter from eastwood with the replaceable cutter end, broke it. then I went to their heavy duty solid cutter... broke it.

finally out of desperation, I grabbed a new 1/4" drill bit, carefully center punched each weld and drilled through the first sheet. about half of the welds needed nothing more to come apart, the ones that did came apart rather easily with a body knife (I used a wide faced chisel) this is what I would recommend