Hows your cam look, If its not pitted and metal isn't rolling on the lobes and the old lifters look good on the bottoms not bowled or pitted you can run new lifters on it, but you must brake them in just like a new cam break in! Dealers and racers and hotrodders have done that for many many years! If it was the case that you cant run new lifters on an old cam, dealers would change cam and lifters every time a lifter was replaced because it wouldn't pump up and made noise! If the cam lobes are wore they will probably wear out the cam and lifters pretty quick!!!!
You Don't need to take the chance. flat tappet cam kits are cheep!