Do you have the 3.23's already or do you need them?when you change gear it affects the converter. Some companies will tune it up for fairly cheap, but you still eat the freight both ways.
I have zero doubt that 284 cam will run in the 12.00-12.teens if the gear and converter. Port what you have, skip ANY dual plane intake and go.
I don't have 3.23's yet, should be easy to get. I did some "clean up" work on the magnum heads, and opened up the bowl area some. Because the mag heads, I'll have to run the only magnum intake I have, which is the Eddy Air Gap RPM.