How about...
since you have chevy springs, you should be able to get the chevy bushings. If the inside is too big (the outside goes in the spring, chevy should fit chevy...)you could use urethane bushings and drill them out - SLOWLY to 5/8. The urethane should be easier to drill than rubber. And teh urethane ones i have seen have no metal insert whereas the rubber ususally does.

Or try a spring 'distributor' (we have a few here that specialize in driveshafts, springs, big rigs, etc) and see what they can look up. Perhaps there is another year chevy that used either pibber bolts or a bushed inside where you could remove the bushing.