Thanks for that Swarfy - very informative.
I have a 356 I bought in a job-lot so I'll put that aside for this engine. It's my first 'big' British bike (after having a Bantam D7 and some japanese bikes) and I'm a mere 25 years old so not too worried about something a little sporty - however more interested in reliability and the ability to cover long distances so a 356 is probably the best compromise.
As another stroke of luck, the crankshaft, cylinders, pistons and top end have all come out of a post DA10 7801 engine so it is thick flanged. Useful to know about the studs though.
Unfortunately, there's a broken fin, and being pedantic I'm planning on getting this welded up and the barrels/head powdercoated before I put the new engine together. I'll also put in new bearings and bushes where appropriate, but hoping I can get away without having to get it balanced.
Tom