There a lot of reasons, but specifically, iron wears well as is. What do You mean by "a little loose"? All exhaust guides are a little loose compared to the intakes, are they too loose, and have You had them mic'd out to see? A little loose is way better than a little tight on the exhaust side, if gives You leeway if You happen to run lean & they heat-up.
A siezed exh. valve=blown engine................
As above.... measure valve-to-guide clearance to see what you really have, unless you have done enough engine work to have a good feel for how loose (in terms of side-to-sdie- 'wiggle') is good, or too much.
