You had me all excited there for a minute Ron. If you read my previous post and look at my call sign you'll see why I get all girlie about early 30's 350 Matchless.
Anyway, back to your bike. The info you want is as follows
Tappet Clearance (Cold) Nil. pushrods should just be sufficiently free to rotate.
Ignition advance: Points break 7/16" before TDC on full advance.
Spark Plug: Original recommendation was a Lodge H14 or KLG F70 so a modern NGK B6HS will do nicely.
Whether your magneto advance is slack or tight on the lever depends entirely on what magneto it now has fitted after 75 odd years on the road. I can only give you official factory information as it was made. Who knows what has been done to it since. Take the points cover off and see what happens when you move the lever.
If you want a little more information on the engine the old weekly 'MotorCycling' used to publish a brief Service Series on various bikes and they did one on the 1937-1939 G3 Matchless. Its full details are Service Series No 12, published on pages 224 and 225 in their issue for June 25, 1953. You can either buy the magazine from people who sell old mags at swap meets or get a photocopy from Mortons or the VMCC Library.
Cheers,
