I am 6 foot 4 about 260 pounds, but I run really well for my size, what position in rugby would I best be suited for?

Almost without a doubt, second row. Your height is wasted in any other position.

second row or prop if needed

second row is a gimme, however I think it would be perfect for you to experiment with the back row so flanker or no.8, take a look at tom croft, kieran read, or billy vunipola they are class players, and if you are quick enough then maybe even try out center

second row