Well Im not the most knowledgable person to answer this but from what Ive read, seen and have in my 12jc it would be an aluminum "A" if its under 500 hp.

I think "A" would be a good call too. 

Thanks rebel that breaks it down alot easier for my simple mind then them damn charts  ya its a stock 455 olds so an AA impeller would probably be my best bet? I would be happy to get around 50 to 55 mph if that is possible with stock 455? Thanks  again 

Food for thought....

An "A" or a "AA" can be cut down to a smaller size. A "B" cannot be cut "up" to an "A".

When I had my shop in the late '90's I would always keep an "A" in stock and I would only cut them down if there simply was not enough HP to "pull it"...

Always go with the largest impeller because you can't "put the shit back in the horse"....


You should do fine with an "A" IMHO.

GT

Thanks rebel that breaks it down alot easier for my simple mind then them damn charts  ya its a stock 455 olds so an AA impeller would probably be my best bet? I would be happy to get around 50 to 55 mph if that is possible with stock 455? Thanks  again 

Completely and easily possible with an Olds to hit that. We got back from Big River with my buds/my  heavy ass 19' Charger that we just put a fresh YJ pump in, with an Energizer kit, A impeller and diverter (The boat is hitting 5000+ rpm on it's warmed over 455, probably 350HP or so, so if I ever trip across an AA on the cheap I think we're going to replace the A with that just for giggles).

But it ripped off 57MPH, popping from a lean condition (Later found to be a weak fuel pump and I also bumped up the jets a couple steps, timing was a little off), but I haven't been back in it to try my GPS on it again.

I think it might be a mute point to argue the A vs AA until you get an accurate RPM reading of what you're pulling RPM wise REALLY. In my case this is an AT converted Jacuzzi and not a true Berk, even though all the guts, bowl, etc are Berk for all intents and purposes, but your results may be different. Simply put, a fresh rebuild on the pump, new "A" impy may well lower your RPM to the 4000-4500 range, saving wear and tear on your engine and hitting your target speeds. hard to say without a real baseline number and how tired your pump really is. This is just based on my experience with this one , My only Olds, but it has had oiling mods done to it, and a 10qt Dooley pan, windage tray, etc, etc so the high rpms probably won't kill it, but I don't beleive it's making any more power at 5000 RPM than it is at 4500, according to the GPS readings we were getting, and the AA might actually pick up some speed while forcing the RPMs to stay down.

I think you'll probably acheive the same results with either an A or an AA, But as Glenn said, it's easier/cheaper to cut one down than to go and buy another one. 
Ray