you can get individual lifters from chevs of the 40's, but I would replace them all, if one is worn, its a pretty good bet they all are.
And Tiny is correct you can just straighten the pushrods provided they aren't really bent over so much to change their length.
Or you can change those only one at a time if necessary, but don't bother if you don't get the valves loosened up, it will just do it again.
