At 345', you're definitely going to have big problems with voltage loss. The voltage from the receivers isn't going to be high enough to run the LNB/switch at that length. To go that far, you'd need some solid-copper RG11, and once you find out what that's going to cost, you'll probably decide to go with another option.
I had a 125ft run from my 722 to a dpplus, with another 200ft from there to the wing dish. I never really had a problem, but it was operating WAY outside the recommended maximums. Please contact a local installer to do a site survey before you settle on this option. Have you considered an Eastern Arc setup?