Looking to replace the stock dual point distributor in my car with an electronic unit. (really!) I have a few electronic distributors laying around, some nicer than others. Is there any difference between a big and small block version other than the length of the shaft? The housings and guts all look the same to me. Any reason I can't swap the shaft from my old grimy B motor distributor into a nice clean small block housing?

Small block rotate clockwise. Looking down the vacuum advance is on the left.
Big block counter clockwise. Looking down the vacuum advance is on the right.
Your complete duel point is worth more whole, don't rob the shaft from it.

yes the electronic B shaft will fit the SB electronic housing. As noted the rotation is opposite so just eyeball the fitment of the vac can arm into the locating hole in the top flat plate so your vac adv is operational which it sounds like you have already done & if good on that system then you're set. While you're at it I would set the shaft end play to .005" with flat no bevel shims above the plastic collar & speedway mtrs has a nice selection of em for $10

Oh yeah, duh. Didn't think of that, thanks. I see it now, the slot for the pickup and vacuum advance is on the opposite side. They looked the same on the bench. Guess I can take it apart and clean it.

But about that Black Flag avatar... I saw them...

the small block and big block housings I have won't interchange.

If removing vacuum, housing between BB and SB can be used on any block. Pickup coil, rotor and vap are the same, Reluctor too, just need to be indexed propperly. Shafts between SB B and RB are diff lenght.

There are non floatng pick up coil plates able to be installed to remove the vacuum pod.

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com%2Fulk%2Fitm%2F190808914361

( these are some interesting tips to use your imagination )

are the advance weights the same with the opposite rotation?

No, if you put a comma at the beginning of a sentence, that is what the SB ones look like. A comma at the end of a word like the one after the (No) mirrors the shape of the BB ones.

Nothing about the shaft, but just the "advance plate"