45-70 groove diameter should be around .456"-.457", H&Rs are usually about .0005" more open at the muzzle than at the throat. For cast bullet fitting, the groove diameter is the important dimension, although nice lands are important too, the shallow rifling makes it more critical to fit the groove diameter plus .001" to .002" for good accuracy with cast bullets. I shoot .458" and .459" in mine, others even shoot .460" very well.
Tim