[Synchro909]

I run a 6:1 head on one of my engines and for a somewhat original look, I wanted to try a B distributor. I had three of them and NONE worked worth a damn so I took one (the best) off to an ignition specialist. He tried but decided it wasn't reasonable (not impossible) to use it so he made up a modern (from a Nissan) distributor with mechanical and vacuum advance features. The result is magic. I have my 22° advance reliably and a great advance curve. He knew what he was doing.
My opinion of B distributors is VERY low. In fact, I think the reputation for B blocks cracking is due to those crappy distributors. I have never seen a new one but after a few years (and a few thousand miles), they don't advance properly. That leads to the motor running retarded much of the time and as we know, a retarded engine runs hot and a hot engine is prone to cracking.
IMO, there are decent distributors available that look just like the A distributor and have a good advance mechanism built in. I think that to use a questionable B distributor is tempting fate and it will come back to bight you on the @ss. I recommend buying a new one with advance already tested and proved. Trying to graft in a B dizzy might sound like an interesting challenge but I fear if you do it, you'll have a bigger challenge in the future - a new engine.