Unless you have an SHO. There is no adjustment on the valves. I would suspect that the problem you are having is no compression when you try to start the engine. This happens because the lifters extend when the pressure from the pushrods and valve springs is removed. You can get then back into their operating position by loosening the rockers until you have clearance. Then tighten then slowly about 1/4 turn at a time to allow them to bleed down. You should bring each cylinder up on compression while you tighten the rockers for that cylinder. Once bled down, the retaining bolts should be torqued to spec.