Erratic timing during vacuum advance

Could be a number of things, but 1st check to make sure the vacuum advance on the distributor is connected to the ported manifold source, not to manifold vacuum under the carb.

What do you mean by "the motor falling off as I gave it more gas. " ? Running rough or?

With KB pistons, the CR is likely gonna be quite a bit higher than stock CR. So this timing be ear and at 18* initial may be too aggressive for that CR and iron heads. Add in a possible 22* mechanical advance and you are pushing 40* total.

BTW. buy a vacuum gauge too.