Bob, I thought of that too (10mm first) but the .40's useful for cheap(er) plinking/target. Think .38 Sp. Truth of the matter is, if I'm (able to be) in that deep into a Bowen/Clements/Harton project, I'd probably spring for the third cylinder anyway, whatever cal it is!...and probably won't "go there" unless I can embellish things how I want them (since this is a highly discretionary idea to begin with!). Starting with a few requisite cylinder blanks from Bowen, etc, my local 'smith is more than capable of most of this, including one of the best tuners around--and if smart I could save a bundle there, and I may pursue to get the budget down--but usually doesn't get into "fine finishing" aspects, re-bluing, etc, and I'd like original new roll markings, a Bowen (Turnbull) or Harton "CCH" etc. I'd rather not be farming this around to two or three to get it done right. So, I my be waiting for awhile yet!
Added - also since I'd be leaving the stock .357 cylinder largely alone (with its mild factory NV chamfer) it'd be the cheapest to just ream out for the .40 S&W, given that I'd like the 10mm fluteless (= custom cyl) just for grins, so might as well go ahead and do the .40.
..And did I say, I shoot .40 already anyway?