The scopes sold by many of the BPCR sellers (Malcolm scopes) come in two sizes. One is a 3x, 17" scope designed for 26"barreled rifles and the other is the longer scope designed for the 30-34" long range rifles.
With a 3x, 17" scope, 34" barrel, and the mounts 6" apart, my 45/70 rifle can be sighted out to approx 300 yds max. According to the distributor, the mounts are curved for mounting on a round barrel. If you have a octagonal barrel, you can file the bottom of the front base and shim the rear base to get some added distance.
I have also seen where some rifles can't hold this type of scope because the hammer is too large. The instance that I saw was an Italian rifle, but not Uberti or Pedersoli.