If I were going to that much effort, it would be a roller when I was done.
As I recall, someone makes roller tappets large enough, it's just that hey re keyed, so you have to drill a raceway for them.
That and they're solid, from what I was reading into the OP's post, he might want hydraulics, not a bad goal, but a properly set up solid should last quite a while; roller should last indefinitely with proper lubrication.
