Phil;

You can (and should) check both vacuum and Centrifugal Advance separately, see info here: https://www.sw-em.com/Volvo%20Ignition%20from%20Scratch.htm#Dynamic_Adjustment_of_the_Spark_Timing
...and you could try disabling Vac Adv/Ret (especially if performance killing Vac Ret!...by just disconnecting Vac Line at Dist, and plugging it at Carb), and performing a road-test.

To answer your other question of "how close reving it in the garage is to running under load. "...it's only good immediately as the revs rise from idle...quickly reving against the inertial of the engine itself will tell if you can expect a stumble off idle while driving, but it does not simulate the engine load at revs very well...there, a roadtest is hard to beat...or a rolling road/dynamometer (but who has one of those?).

Good Hunting!