18* initial sure isn't much with any cam at all.
If it were mine, I would want more initial than that. And it seems to me that the best route would be to shorten the advance slots. Can someone explain why manifold vacuum would be a better solution in this case than more initial in the distributor?
I think the main concern is whether the engine can crank and start with too much initial.
If the engine likes 20+ initial timing, but can't crank with that much, pretty much the only option is to use manifold vacuum to give a lower initial, or some sort of start retard box.
If it does crank fine with 20+, then sure, limit the advance slots, and set the initial at 20+.
Also in general, the engine vacuum at idle needs to be higher than the vacuum in the advance (if using manifold). If they're similar, then the timing will change with vacuum, which changes the vacuum, which changes the idle speed, which change....
It will never settle down and be steady. Similar issue if the step up springs in an Edelbrock are the same as the idle vacuum.
