The B engine has a bigger intake port than the A engine. I've been told that the port timing between the two is the same, but the port area between the actual port window and carburetor mounting flange is bigger on the B version. I've got several of both engines and I've been going to measure the port heights of an A and a B just to verify that they are the same, but haven't done it yet. As far as I know this is the only difference between the two. As far as value, guys that are building one lunger racers for the stock class would probably prefer the B model, as they're rated at 22hp. The A model is 20hp. But if someone is restoring a sled or if they're building a mod class racer that's going to be ported anyhow, I'd say their isn't much difference in dollar value between the two engines, IMHO.
Thats about right, but ive only seen the a rated at 19.5 and the b rated at 20 so im not all that sure, ive got both and i prefer the b as it has more power but like you said mod racers are ported anyway so.