As I've posted before, FBO's slot limiter is clever but is not the best way to set it up. The better way to do it without a Sun machine and springs is simply weld up the
inside of the slots and reset the initial timing. This returns most early smogged curves to a presmogged setup.
318 is pretty efficient engine at lower rpms so not going to need as much advance at low rpm as a cammed motor. Don't think your going to be looking at a major difference - unless maybe it had one of the funky advance solenoids on it.
Here's a graph to illustrate both points made above.
Timing for a '68 318 without CAP vs a the timing for a manual w/ CAP.
1. There's not much difference. Mostly in the beginning. From 1400 rpm up its the same.
2.a Shortening the outside of the cam slots stops the secondary curve before its fully advanced. When the initial is reset, the whole curve may be too early. Then you have to monkey around with the springs or there may be part throttle pinging, etc.
b. Shortening the inside slots means the timing will look pretty much like the non-smogged when the intial is set around 5*. Done.
Note: 1970 CAP/CAS curve may be different than '68
