What have you done today with your Austin Seven

[Bob Culver]

The limitation of the early heads is the restricted aperture into the cylinder. From reported performance of stock RNs cf RPs I suspect the raised cr of the latter might actually reduce power at revs. Wedging raises the cr without greatly reducing the passageway.
Apart from being lower cr the early heads lack the violent squish which much hastens burning and hence engine roughness, esp evident with the 2 brg crank and later head.
As for original crank life, fatigue is enormously influenced by peak pressure which is roughly prorportional to cr. Under constant test conditons fatigue shows considerable life scatter. With varied loading the randomness is vastly increased, so some cranks survive remarkably. The stack of cracked ones under my bench were destined not to.
(Curiously early overload can greatly increase fatigue life. Might not apply to nitrided which have high fatigue resistance anyway)