Have a look for the 'Original AC Ace and Cobra' book by Rinsey Mills. ISBN 1-870979-14-1. The book includes a guide to the Ace powertrains. Cost wise, yes I think the Bristol is very expensive as are the parts, there are occasional Bristol engines on eBay, but in need of a complete rebuild and not of 100D2 spec.
I haven't ever drive one, but as I Ford employee, I'll put a mention in of the Ford 2.6L 'Zephyr Six' engine, these were bored in period to 2.8 and are mush less expensive. I did see a freshly built Zephyr engine on Race Cars Direct which with a 12 port Mays head, but without carbs or manifolds was about the same as a Bristol needing complete rebuild on eBay. So in my mind it's a very cost effective period engine, that could perhaps be a 2nd engine if you bought an Aceca with the AC engine.
Regards James