I rigged up some welding electrodes from copper bar and rod,made a fire button to manually turn on the big SCR,found some thin stainless sheet metal about the same thickness as the battery cases.

The first welds were not strong enough,and could be twisted off with only a tiny weld zone that was not sufficient, so I got some 1/4" X 1" copper bar to lower the resistance to the work and between the capacitors, dug out some more capacitors, and made a new capacitor bank.
The new cap bank totaled 790,000Uf and can be charged to 25V, yielding 247 Joules or Watt seconds. The weld penetration looks complete across the electrode tip width, and is slightly burned, so we have more energy concentration than required, so a lower voltage or wider tip may work even better.
The welds are quite strong, so we may have sufficient energy for welding the cells, but a bit more Capacitor, maybe a total of 1 Farad or so would be better.
This would allow operation at lower voltages and will leave the base metal in better shape.
Will put out some feelers to see if I can scrounge up some bigger Caps.
Next I need to make the cell holding and weld electrode assembly and try welding real cells.