Valve seals tore on break in

I suppose that's true. I work at a machine shop, so I do it anyway. Old cast iron heads weren't machined close most of the time so I like to set everything where it is equal & usually cut .100 off the top of the shortest , then match the rest. I also cut the guides od to .502 on all cast iron heads. The new guides I install are that size. I also check what cam lift is just so the clearance is there.
I used those same seals on my slant 6 head and I still have a LOT of room between the guides and retainers. You probably weren't thinkin about a slant 6, though. lol

BUT, I did use 318 valves for some valve spring. They are .300" longer than slant 6 valves.