Keith,
I put threaded studs in the top and bottom bars and then threaded the anvils before I heat treated them. I just bought a piece of 1 inch round A2 to make them from. I have tried various shapes and find that a very slight concave surface slightly larger in diameter than the pin works best. If you pre-shape bolsters as I do, then too large a diameter and you wil press dents into the bolster because of the curved surface and the overhang. It really does work well once you get past the learning curve.
Randy