Hans
In my experience, the best way to time your six is by the seat of the pants. If you have the vacuum retard operational, strobe it in to around 4 BTDC. Then take it out and find an incline to labor the engine in fourth gear. Advance the distributor a wee bit and climb the incline. If you don't hear detonation (pinging), you're not quite there. The objective is to advance to detonation, then back it off slightly. Of course, you'll have to adjust your idle when you get to that point too.
Good Luck!