Can you explain this a little further? I thought that an R block did not need bushings, or is the lifter bore height similar to a stock block? I assume you are concerned about how much lift you have and exposing the oil band?
Blocks the oil pressure needed to use hydraulic lifters. Not really needed for a solid lifter, or a solid roller cam.
You can lose a lot of oil pressure through the lifter bores that’s needed for the bottom end, main and rod bearings.
