Sorry, can't help exactly, but I'll tell you where you can look. Try Jack Collins website on the Crossflow Chronicles conversion to his 1966 'Stang. He had the exact measurements, I think. From memory, it was the same diameter as the US 200 I6
Note 1 "I will ream the block out to use a Duraspark distributor tomorrow. The block is machined for a .485" distributor shaft. The Duraspark shaft is .515"
Note 2 "I reamed the distributor shaft opening to .515" in order to use the Duraspark. When I placed it on the mockup, the distributor turned out to be too tall. The cap hits the intake runner. I will have to use the Duraspark internals in a points type body. "
Apparently, this is a common problem as the cage of the EFI intake runner mounts are very close to the dizzy. There's little space in this area on any crossflow 250 engine, (iron 1976-1980, alloy head I 1980-1982, Alloy head II 1992-85, or any EFI..'83 to '88).
Second thing is, there were two types of EFI distributor. The early 1983 to 1985 XE variant had a non EEC4 module, it was just a conventional Bosch number with old fashioned concetions, not with plug connections. They are cheep and plentifull. They second type, much better, is the 1985-1988 XF EEC4 type which had spark plug connections on the dizzy cap.
Use what Jack uses. I think the US cap and Duraspark II ignition looks pretty hot, but my guess is you'll have to mix in match the earlier components from a non electronic 200 I6