Just as a practical matter, small block V8 valve springs, retainers, and split keepers are rated for much higher RPM than an unmodified stovebolt six can ever run. 350 Chevy V8 exhaust valves are an exact dimensional clone of the stovebolt valve. I've run them at 7K+ RPM for thousands of laps on dirt tracks. Compare prices- - - - -even stainless aftermarket valves and the associated hardware are dirt cheap. I've used them by the dozen on plenty of stovebolt heads with no failures whatsoever.

www.ebay.com/itm/202799964937?

www.ebay.com/itm/800856779636?

You'll have a few leftover parts- - - - -so what? Give 'em away! (Or stash them for the next engine build)
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!