After speaking with one of our knowledgeable members I decided to sort of line bore my new plain bush, locating the reamer accurately from the drive side bearing outer.
By a complete stroke of luck some time ago I was given a brand new adjustable reamer and a large lump of brass which was used to make the bush / reamer guide.
I also made a plug gauge with three progressive diameters to assist in getting the correct size.
Long story short, it seems to have worked; on assembly, the crank turns smoothly, and a light covering of engineers blue on the journal showed up its evenly in the bush. Obviously, proof of success or not, will only show when the engine is run, and as you can imagine, I'm crossing everything I've got! On the crankshaft drive side where the shims locate, I noticed a ridge which was preventing the shims from sitting flush, so I also set the crank up accurately in the lathe, removed the ridge and lightly skimmed the web to provide a good, smooth surface. Preliminary assembly is showing end clearance at 0.0015". Final assembly will start when I get the Wellseal I've just ordered.