This is always a tough one with sportcoats...as much as measurements and your stats help, nothing tells the tale like trying the jacket on. I've learned a ton through my bespoke experience, and befriending my tailor, and this is one of those things. If you can get into NYC to do this I would. If not, maybe you can find a cheap one on B&S ...
macjedi - I've ordered long size jackets from Epaulet, and the whole pattern is changed to compensate for the extra length. So if you're worried about balance, you can rest assured that it won't be just a regular sized jacket with an extra inch lopped off the overall length of the body and arms. However, if you're worried about the rest of the fit, it would be best to try on one of the stock jackets and go from there.
As Beatlegeuse said, if you order a short or long, the entire pattern is adjusted. Same if you special order an in-between size (I wear a 37 Napoli, for instance). One thing to bear in mind: HY's shoulders tend to be broader than Epaulet's. An HY 36 has a 17" shoulder, which is the shoulder measurement on an Epaulet 38. If you had a little room in the shoulders with the HY, an Epaulet 36R might be perfect for you anyway. Matt, Mike, and Adele are always happy to give really detailed sizing advice over e-mail or over the phone.
Thanks for the great comments fellas! I'm much more confident. I really want a lightly constructed hopsack, so I think I'll try a ready-made before I go for the MTO.
Sorry if this is already been discussed in detail, but is the OTR Caine equivalent to the Southwick MTO half-canvassed 3/8 lined option? And the Napoli equivalent to the Southwick MTO unconstructed 3/8 lined option? Just trying to get my facts straight. :)