Partly because roller cam retrofit to a non roller cam block is more expensive. Solid roller will be most expensive. It all depends what you have planned for the car. If I was using a 318, I would use a hydraulic roller cam. If you use your 86 block, just buy a cam for that and stock replacement roller lifters. I think a solid roller would require a consumable brass distributor gear. Call the cam companies and find out what you need to do. I'm just a peon here. Wait for a few other people's opinion.