A later crank can be fitted in an early ('65) case?
You just need to make up a couple of thrust washers.
For me, that's just asking for trouble.
The misconception about the "narrow" or "wide" crank is that the entire thing is wider. This is incorrect: only the internals, shoulder-to-shoulder, are wider. The crank as a unit it the same width, tip-to-tip.
What this means is that the later cases are machined on the right side to house a slightly different bush (plus thrust washer) and then the left side face is machined thinner to provide room for the wider counterweights and (this is critical) the bearing lip is machined to shift it to the left
, away from the crank.
Once you've gone through the headaches of actually getting a later crank to fit in a narrow case without binding or hitting anywhere, you'll next need to identify the correct spacer(s) to realign the crank sprocket to the clutch chainwheel, the alternator rotor to the stator, etc.
I'm not saying it can't be done, or that is hasn't been done successfully. I just feel it's much more likely to go wrong than right. I personally would never be confident that I didn't get one of the measurements wrong and would feared having the whole thing lock up when warmed up. Much easier to stick with proven parts.