Steve,
I started out looking for a 66" boxblade for my 4310 also, couldn't find one in stock at any of the dealers close to me. I ended up buying a 72" Frontier, and am glad I did.
The extra width makes it much easier to get the blade up close to fences, buildings, etc., and also does a good job of covering my tracks when turning fairly sharp. The extra weight also helps in hard soil.
I also have the largest R-1s on mine, set as wide as they'll go. The 4310 has no trouble pulling a 6' boxblade.
See if you can give one of each a try. I definately think a 5' would be too small.
Good luck,
DT