The standard twist for the 38-40 in a rifle is 1-36". Unless you're planning on shooting unusually heavy, long bullets (the "usual" bullet is about 180 grains), that should be fine. You don't mention the exact twist of the rifle you're looking at, but I'm guessing it's 1-36". Most of the bullets available will tend to be pretty short compared to their diameter, so they don't need a fast twist to stabilize. Besides, a slow twist can be a little easier on lead bullets, which I'm guessing is what your gun will be digesting. Some 38-40 revolvers do come with a 1-16" twist, but all the rifles I can look up use the 1-36".
