Try 18-20 and see what happens. You'll have to bust open the distributor to limit mechanical so your total number isn't out of bounds. On a street car, total timing method is a horrible way to set up the ignition. On a race car that sees pretty much only full throttle, fine, street driver no way.
Stock 318's like 10-12 initial timing, you have a much larger cam and are at the same level. That's not going to make for a happy idle profile. Think of this as building a house. Total timing is the roof, initial/idle is the slab. If the slab is fubar'd, the rest of the build is going to need adjustments/crutches to get the roof line square.
Simple test. Get the engine warmed up and idling. Give the dist a tweak clockwise... if the engine picked up rpm, it wants the advance. My guess, a xe274 in a low compression engine is going to want 18-24* initial timing. It should idle at 800-850 with minimal drop when pulled into gear. If you are getting a large drop in RPM pulling into gear, that's another indicator of insufficient initial timing or dropping timing out from reduction in RPM.
Put a light on it, have somewhere pull the car in gear and see if the timing is reduced in gear from where it idles in P/N. That is an issue. You want the timing at idle to remain the same in gear and out of gear. This pops up a lot when idle speeds are around/above 900 or so with a lot of distributors with light springs in them.
