Getting the additional barrels factory fitted is just a matter of shipping the frame and stock set off to them with an order form, put on the order form that you'd like the trigger pull reduced to 3-3½lbs, wouldn't hurt to put a note or tag on the frame to the same effect, then at least the gunsmith will know that's what ya want in case they don't put it on the invoice.
http://hr1871.com/Support/accessoryProgram.aspxThe BC carbine forend is shorter, but the standard forend would work fine, you could also order the carbine forend at the same time you order the barrels, it's $31.88, part #R03944.
If you get the 38-55 barrel, it comes with no sights, be sure to order them if you want em, their price on the WGRS is $18, about half of what it is anywhere else.
The 38-55 is a good shooter with the right cast bullets, jacketed can be a nightmare since they're too small in diameter for the overbored barrel, at least so far anyway. Handirifle is shooting .379" cast in his, much to his joy, and no problems with an undersized neck in the chamber, which used to be an issue.
Tim