"Jeez man, this has been done to death over the years". "Search the boards". etc etc ad nauseum
260/270 cam = o.e valve clearance (inlet: 0.15mm, exhaust: 0.20mm). (*)
285 (or Kent rt150) = inlet: 0.20mm, exhaust 0.25mm.
300 (or Kent rt200) = inlet 0.25mm, exhaust 0.30mm.
I very much doubt anyone could get them exactly to those clearance dimensions, so being 0.05mm or whatever out isn't really gonna make that much difference in the grand scheme of things.
* Awaits someone to argue that o.e clearances are inlet: 0.20mm, exhaust: 0.25mm...