You need to machine them to give the proper valve spring installed height that your springs call for. Find that spec from the spring manufacturer and take the heads to your machinist along with the retainers and locks and he should be able to figure it out easily enough. When he does it the guide gets cut down at the same time and you have to run poly lock valve seals. With mine I measured the stock installed height at 1.68" and the springs only called for 1.7" installed height so he didn't have to take much off. Mainly had to take the guide down so the inner spring would clear. If you plan on going with a pretty big cam you may want to look into springs that are for a higher installed height like 1.8". That'll give you room for a pretty big cam.
