If you don't know how much centrifugal advance you have, I don't recommend arbitrarily setting the initial advance to 14° BTDC. You could easily have too much advance at higher RPMs, especially if the factory recommended 5° ATDC for initial timing with your distributor. With LPG's high octane number, it is possible that you could be very over-advanced and never hear any pinging so never time your engine by ear. Over-advanced timing can cause your engine to run hot. Remember 65CrewCab is only running 22° @ 3000 RPM of total mechanical advance in his truck.

If you can't find an adjustable timing light, I would mark the damper for 28° and 42° to see if you're in the ballpark. If you're way out, you need think about recurving your distributor and there are other members here that are far more knowledgeable about this than me.
http://www.slantsix.org/forum/viewtopic.php?t=50214
http://www.slantsix.org/forum/viewtopic.php?t=38471
http://www.slantsix.org/forum/viewtopic.php?p=73272

If you've got the budget for one, I would also invest in a wide-band O2 sensor. It is important to get the fuel mixture correct as well as your advance curve.
The Propane fueled truck...

If you take your car to a local mechanic who isn't familiar with slant sixes, you could end up paying a lot of money without having anything to show for it. Be careful.

_________________
1965 Plymouth Barracuda,
225 engine, Quadrajet, HEI, Dutra Duals, 904 Torqueflite, 2.76:1 axle, Addco front bar
Rods & Relics - Fort Erie, ON / Collector Car Tech