now with that said, i have no idea how .0025 larger diameter retainers would fit the 5/16" valve stem tolerance wise. or the other way around. plus, how the retainers would fit the spring retainers being used.
it "could" work, or "not" work, caused by however the retainer grooves were cut in the stem, and how the holes were machined in the spring retainers.
without knowing the +/- tolerance on the valve stem grooves and the same on each type of retainer, i would not recommend interchanging the two types.
that is just my 45+yr machinist career talking, and NOT any experience interchanging 5/16" and 8mm valves and retainers.
also, i'm not bashing any engine builders, as they have WAY more experience than i do !