Flathead Distributor interchangeability

I recently purchased an 81A and its got the "eggshell" dizzy, I have a lead on a Mallory "crab" capped dizzy and was wondering if this will bolt on to my prewar flattie or if there is a difference between the timing cover, etc on flattheads with the "eggshell" distributor versus the later "crab" version?

 

There is an adapter for this. I believe Speedway Motors sells them.

 

Yep, you need the adapter to go from a 3 bolt dist. to a 2 bolt dist.

 

As stated above, an adapter is needed for two purposes, to adapt the bolt pattern obviously, but also to space the crab distributor further away from the cam, as the earlier cam had a longer snout.
If you change to a short snout cam, you can just fit a two bolt front cover (covers are interchangable) and run the crab as intended, no adapter.

 

Thanks for the responses, think I'll try get an adapter as from what I can tell the later crab style is a better option compared to the eggshell variant.

 

Mallory crab? I have seen many of the 21A style crabs but all were made by Ford. I just had a problem with a replacement crab. My original on an adapter was partially throwing off the retaining ring for the point plate(common problem). I could not get it to stop, so I bought a Ford remanufactured crab in nice shape, tried the fit onto the adapter and the engine and sent it out for another rebuild and time. When it returned I installed it and the car would not start. I removed it to find the entire back end of the distributor housing was ripped off. It appears the housing around the rear bushing hit the front of the cam, when the cam rotated the damage was done. I put my old crab back on and the car started fine. I then sent the crab out and informed the distributor shop about the snap ring problem. They found the point plate distorted and replaced it and everything works fine, I did install thicker ( from .015 to .025) gaskets between the adapter and the front cover and the distributor to the adapter. One other thing about the adapters, the threads to mount the distributor are fragile and over the years will probably be damaged, as mine were, probably from someone using bolts that were too long. I drilled the two 5/16-18 holes out and my local machine shop supply store had steel thread inserts that with some mod worked well.