It seems that you should bore a 1 7/8 hole in the end of the roller shaft, that will keep it centered and straight. I do not know if that hole could easily have a keyway added to it, machine shop question.
Or my other thought is to weld a flange on the end of the 1 7/8 machine it square to the centerline of the shaft and bolt that to the roller. I would want the flange to sit is shallow recess on the end of the shaft to keep it centered, maybe a 3 1/2" flange, or put a 1" hole in the end of the roller and have them leave a short 1" spud on the end when the flange is squared up. I hope that makes sense.
Sounds like your rollers will be quite nice.
smith out
I think I like the bolt on flange idea best, if you ever need to modify or upgrade it is easy to do compared to having a welded on shaft.