[chris vickery]

Richard, as you have seen, the change over from a diver helmet to a crab style is relatively simple. It's just a case of having the correct timing cover to match your dizzy and the addition or deletion of the spacer plug.
The early 32-41 block used the 3 bolt type mount whilst the 42-48 used the 2 bolt type mount. The crab is a much better system with the use of the remote mounted coil which keeps it a lot cooler and not as subject to thermal breakdown as the diver's style coil.
There has been a lot of development in the hot rod world with this stuff, I see that there is now crab style dizzy available with built in electronic ignition.
As posted by previous fellows, the changeover from a late model 48-53 block is quite an involved task. Aside from the engine going into a 12 cab with limited room, most would ask why one would go backwards with the technology in place. I believe also that Merc camshafts can add another dimension into the already confusing scenario due to changes in dimensions as well.