I have a 500+HP 454 in a 73 stock Caprice Conv. (i.e. as in heavy).  Not at all happy with the engine to rear wheel transfer.
Questions - reccomended stall converter stall?
rear end gearing?
I have a used 10 bolt posi from the same year vechile I guess I am going to rebuild.
I have stock 15" wheels/tires but need to replace with either 16 or 17" wheels/tires.
I would like decent gas mileage, but also a decent hook up from a dead start.
Suggestions??  

You do not need to get 16 or 17 inch wheels or tires. Are you spinning the tires when you take off, or is it sluggish? What cam is in the motor? radical? Mild? what rpms does the motor pull? this will determine your stall for the converter. What gearing is in the rear end? how tall are your rear tires? What transmission do you have?  

Not to rain on your parade, but, You wont get decent gas mileage from a 500 horse 454!. really though, with the info super asked for, we should be able to brainstorm some good iseas for you!!