the stems, while advertised at 8mm are actually 5/16". chrysler 2.2L/2.5L retainers work with a 1.4" OD valve spring and stock magnum keepers. otherwise any aftermarket setup for 5/16" valves will work. Hughes sells some for under $70.

watch out on installed height--magnums are notoriously short on spring installed height. my OEM magnums are only about 1.64", the R/T heads were known to be worse. a lot of LA style springs have a coil bind height of over 1.1", so you'll be very limited on max lift. the Hughes #1110 spring works very well, because it has a coil bind height of 1.040" which will give you a comfortable max lift potential of .525-.550"....