I think you would be fine with the 250's. But if money is not an object and you often run in non-open waters I would strongly consider the 350's.
I just bought a 06 Marlin with F250's and took a ride on Easter Sunday with about 200 gallons of fuel and eleven passengers (yes eleven including kids and myself). This was in the bay that was pretty calm that day. The boat easily cruised at 30 knots and topped 39 knots at WOT with all that weight. I guess the total weight of boat, motors, gear, fuel and passengers was around 14,000 pounds. I was impressed with the numbers, so was the crew.
I, like most others never complain about having too much power. But for me the 350's would be overkill for 95% of my trips. I guess when I am fullly loaded with fuel, a crew of six large fisherman, 150lbs of ice, six flats of bait, chum, food, drinks, etc. and the seas get steep (like five or six feet) I would wish I had the 350's for running over the occasional six footer.
Being that your boat is a little lighter, I would think you will use less power than me at the same speed. Lastly, will the boat go slow enough on those nasty days that we are faced with once a year with those 350's (and those roughly 23" pitch props)? In other words with 23" props will the motors allow you to run at 18 or 20 knots in an attempt to get in safely. My guess with all that torque of sixteen cylinders and all those cubes the motors will do whatever you ask of them.
Just my thoughts.
8)