Thx to all for your input. The car is quite loud so detonation/knock is hard to hear! It does have a long duration cam and single-plane intake so I know it can use more advance than I have with the all-mechanical that's in it now (35 total). I know it could safely run something in the low-mid 50's and was hoping someone sells a bolt-on piece to limit vacuum advance. I'll look at the can/arm to see what it's stamped. Sounds like a little old-fashioned welding job or set screw or some type of mechanical stop on the vacuum mechanism is in order.
