Valve stems are easy to shorten and new keeper grooves can be cut on a lathe. They are NOT hard as if brittle they would snap in two. A motorcycle shop used to bring us 390 Ford valves that fit some old Harley and we would shorten them---a lot, and cut new keeper groves. On race engines we got long stem valves and cut them so the rocker roller was on the exact center of the valve stem at half lift. Using off the shelf stuff sometimes had the roller running off the end of the stem and onto the retainer.
