[wtm442]

Distributor vacuum advance line should be attached to small port, usually located just beneath the primary fuel bowl on the passenger side.

Timing at 36 º at 2500 RPM and above with vacuum line disconnect at distributor.

I am not familiar with MSD. Could the 21 º bushing be "distributor degrees" which would be 42 º damper degrees of mechanical advance?

If you are having 50 degrees advance with vacuum line disconnected, I would switch bushing to a 10 degree unit (10 degrees distributor). That would be 20 degrees crank mechanical advance. With the 15 º initial timing, total would then be 35 º total.

Of course, check everything with timing light.

Good luck. Your 50 º+ could hurt the motor. Make that will hurt the motor if you add laughing gas.

Of course, if you are having 50 º with vacuum attached at a steady RPM, that is OK, and your miss is being caused by something else ... weak coil, bad plug wire(s), time for new spark plugs, etc.