318 hydraulic lifter preload.

The 'thinner' depends on if the surfaces are reasonably flat (see the other on-going thread) and what your present CR is. With stock 318 pistons of that era, you'll don't have a lot of CR to start with so no real worries with CR for your case.

And agreed on the 'no oil in the lifter making it easy".

If your valve tip heights turn out to be low, then the seats need to be turned in deeper. But that may be a lot of work. Instead, I would consider getting the heads surfaced; it will help sealing and CR. Then if need be, I'd get a new set of pushrods after proper measurement. Can yours be returned?