Ah ! Two 400 motors would be quite a load for any battery. As the boat is quite large, then you could go for a sealed lead acid type.
Depending on the weight allowed and the room inside it is up to you. I always use 12 volt, 7 amp hr types (mainly because I have 10 of them lying around)
Another trick (if that's the word) is to have the motors on separate control sticks and just cruise around the lake on one motor, gently. This will give you a much longer time at the lake. I use this idea on my Gry Maritha and find one motor will propel the boat nicely. If my rubbish driving gets me close to danger, then I can push the other stick and power out of trouble.
Just a thought. 
All the best and thank you for your PM.
Cheers
Ken
Not in a Fletcher. A pair of 385s will do the driving, the batteries need to be a good shape to sit low, Fletchers are tender. If going for a 6 cell pack, 3000mAH will do the job, but it would be a good idea to check out the plethora of recent threads on these, both the Lindberg and the Revell to see what others have done.
As the Lindberg was originally intended as a working, but non radio model, it will probably have the bits to make a battery box, probably 4 D cells. A 6 pack 3000mAH pack will probably weigh about the same. Run flat out continuously, probably a lot less than 2 hours, cruising, probably more.
