Rucker's were made when the Model A was new. They are somewhat rare now. They were similar to several OHV heads of the day, like the Cragar heads they could use stock A manifolds or aftermarket manifolds to suit. Miller, later made by Cragar, advertised that their head would double the A horsepower when a slightly larger carb was used.
Cragar's, and likely others, used 6 cyl. Buick rocker arms, 1.25:1 ratio.
There were at least 50 OHV conversions made for the A/B engines back in te day, and some are still being made today. The ones being made today are much improved, I've seen 225 HP on the dyno, and that engine was built as a touring engine, not a racer!