Re: hydraulic lifter problems
A factory Jeep straight six doesn't require valve adjustments, but when you mill the head or block, change the cam to one that has a smaller base circle, cut the valve seats, etc., you may find that an adjustment is necessary. Aftermarket cam companies sell shim kits, adjustable pushrods, and roller rockers that fit the six and require/allow adjustments to be made.
You don't need to prefill the lifters before installation, sometimes this will cause the valves to hold open until the oil thins out and lets the lifters bleed down. A coating of assembly lube is all that is required.
Hope this helps, good luck!