OK, couldn't wait, so I pulled the dizzy from the 1275A+ today for a look...
Dizzy body is the same diameter and length as A series, drive dog is different, as is the driveshaft in the block.
Roll pin diameter is much smaller than A series but is in about the same place.
Maybe the Pulsar pin will fit??
[Best way to mount it in an A+ I think is turn the dizzy around a bit, and put the A+ clamp bolt thru the dizzy's slotted flange. Slot will need to be made wider. Probably the driveshaft will need to be turned a bit too to get timing set right].
Good idea (not), see below..
<edit>
OK, I checked out putting the Hitachi dizzy in the A+ block today. The mounting flange is about 180* from where it needs to be, to use the stock clamp bolt. Vac canister hits the starter with present flange position, so that method is no-go unless I slice vac canister off and run without..
Body length and diameter is OK, shaft just needs a few mm ground off the end and an A+ drive dog fitted.
I think if the Pulsar flange were trimmed off the dizzy as per the mod for an A series, a little filing of the ribs on the body will let the stock A+ clamp finger do the job.
I'm not fitting it in until I can get another A+ drive dog. I'll keep the A+ dizzy intact.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
