As ToraDog says those motors are far too high a kV, something around 1500kV would be more like it, especially as you are running two motors. Lower kV motors would be shorter and therefore lighter.

My opinion, is to have two Lipo Packs side by side, with 'Y' connector to the motors.
You will double your running time.

Yes you could use smaller, slower motors. You have a lot of weight to install, 2 ESC's receiver, ect.
before you get too far, I would put all the components into the hull that need to be there. Not installed, just as weight. I would put you hull in a tub and see where she sits reference her waterline. You can go deeper than scale, particularly if your motors will get you onto a plane, as ELCO's usually ran, but be careful. There is not alot of difference between a bit more and getting sunk.
A couple of other thoughts, 1) check around on this site and other model sites. You are not the first to build this model as a RC conversion. Check out how others have done their builds.
2) If you have a RC plane background, try to determine the average flight times for your motors and battery configuration. Mind you that you will be drawing somewhat differently because of two shafts off one battery.

Thanks for the reply, as you may see from the pic, I've already installed the 2 props so I assume I can use smaller motors thus giving me longer run times?

If my research is correct( don't hold your breath) that motor, with that battery, will turn 33000 at no load.
Honestly, I think that ONE of those motors would be more that enough to power that hull at greater than scale speed. Running one motor will give a long run time and simplify your installation. As long as low speed maneuvering is not too important to you.