I'm in the process of converting mine from wire/rope to all-rope too.
Based on comments here and elsewhere, plus the bad condition of my sheaves, I had new ones made by Zephyrwerks too. The machining was beautiful, and the cost quite reasonable (under $40 ea for quantity 4). They fit perfectly, and spin quite easily.
I had them grooved for 3/8" line, based on AnaMeg's comments and my measurements of the sheave box width.
Elsewhere, I've seen negative comments on Sta-Set X's stiffness, so I asked around my experienced cruising friends. They all said to go with VPC. Defender has it in 3/8" for $1.34/ft. In my dry fit testing of the masthead assembly, the lines pulled freely with no noticeable drag. Hopefully, I'll find out how it all works together when I get the mast finished and back on the boat in the next week or so.