Andy Culpin was ahead of you there AMCO, but I think he has changed tack and now wants to do a Sebring MGC instead...
We used to have problems with Jim's car, as the MSNZ manual was contradictory. On one page, it stated that original block and head materials had to be used and on the next, that period mods were allowed. With an alloy crossflow head as a period mod, it was tricky.
Jim's "must win" attitude was why we changed the BMC/BL Series to an invitation only one and two handicaps. Sorry to say, but the finger pointing and "illegal" engines were common even then, and much as people would like absolute authenticity, enforcing it is all against the spirit anyway, so choccy fish racing (may change the name of our Series...) is better to be self policed by the drivers and if one 1957 Wolseley 1500 goes like stink and a Riley 1.5 in standard trim is heaps is slower, so be it. It is all a bit like cheating at golf, a bit pointless really unless you are up front about it.
I'd be rapt to see a pre 1960's saloon's grid out there at Roycroft even if the cars weren't 100% kosher, as at least the spectators would know what these older saloons looked like, even if not fully au fait with the finer points. We don't want V8 Magnettes, V8 Zephyrs, Morrari recreations out there, but twin carb A35s and Morris Minors or even triple carb Zephyrs and Westminsters I have no problem with at all.