Why mess around with these un-sanctioned substitutions when you can just order the part. The o-ring may have cost a few dollars, but it was certainly less than $10.

When I did this replacement, I got two o-rings, but I think I only used one. Since you have the solenoid shutoff valve, yours is probably the same as on my '73.

The P/N should be: 000 071 2160 $4.20

I removed the carb to change this which may or not have been necessary. There is a spring-loaded piece on top of the solenoid shutoff valve, and I'm not sure if that stays in place if you remove the shutoff valve entirely. So you might check that out.

Anyway, you might also consider getting the float bowl cover gasket as well just in case.

P/N: 000 071 4680 $7.80

Incidentally, that o-ring gets dry over the decades and tends to leak or get torn easily as you saw.

Be very careful with the solenoid shutoff wire as it is aged and very fragile. You must disconnect using the single pin connector that may be tucked under the air intake setup. Disconnect and coil it up carefully before turning the shutoff valve or the you'll need to do surgery on the shutoff valve to fix a broken wire.