1971 slant six 225 no spark or weak spark at plugs?

Use another battery [ such as your every day driver ] to power the ign.
Remove ALL wires from the coil [+] terminal. Other battery: connect [-] terminal to Duster battery [-] terminal. Other bat [+] terminal: connect to coil [+] terminal.

If no spark, you have an ign problem. If spark, but no spark, mechanical problem such as ign timing, valve timing/adjustment.