magnum valve train questions

There no "other retainers", what Hughes sells is it, unless You want to install longer valves.
Magnum lock grooves are very high on the stem with a short tip length, Hughes removes some of the retainer meat below the locks & fattens it a little for support, .540" lift is it unless You;
A) Cut down the guides
B) Use longer stem valves.....
which are 5/16" BTW, not 8mm,
so if You swap valves 8mm will require resizing the guides & different locks etc.