Which Slant 6 to build?

68-71 head would be my choice. Hydraulic lifters sound great, but then it don't take well to more compression. The most popular way you get more compression with these engines is to mill a certain amount off the head and/or block, typically ~.100, but measurements need to be taken. Taking that much of tends the wreck lifter preload, requiring new custom made pushrods. Mechanical rockers have adjustable lash that can take the .100 cut.

Also, it seems like the consensus over at .org is hardened valve seats were a solution to a problem that may or may not have existed in the 70's but isn't much of a problem now. In other words, you can go without them and not have a lick of problems.

Just something to think about!