Look in the tech archives for plenty of 500 inch stroker builds.

http://www.moparts.org/Tech/Archive/bigblock.html

I also wrote an entire book on the subject which you can find at various Mopar parts dealers.

I don't think you'll need the 3.91 gears if you build a 500 inch engine. 3.55 will work just fine as will 3.23 gears.