While you're in there replacing the brass blocker rings, do a close inspection of the splines where the synchro collar meshes with the gears. I like to use a magnifying glass to inspect the ones I repair. If there's any wear or chipping evident on either the internal splines on the collar, or the external teeth on the part of the gear that meshes with thw shift collar, the blocker ring swap won't help much. If it's been grinding very long, there's almost ceertainly a little gear wear or damage to deal with.
Jerry
