If you want the best,
get a Lee built box, SC&C carries them. The open trackers swear by this guy (IIRC his name is Tom Lee), and he's been upgrading boxes for race teams for years. I plan on using a 670 Lee built box in the 67, mainly to buy some extra header room over the larger 800 box. That would save you a few pounds. If you do go to that degree, talk about the valve in the pump too.
Otherwise an AGR or even a reman IROC/WS6 box will work. Although reman boxes can be hit or miss since they all look the same. Most people like the AGR boxes, although like any product you'll see a handful of complaints on the web.
Rebuilding a box is no small event. There is no real benefit to changing over to R&P, except maybe some weight.