My personal pick for heater are Raypack, very reliable and quiet while running. the 336,000 btu model will work. If you are kind of lazy with water balance get one with a cupro nickel heat exchanger.
http://www.raypak.com/jumpPool.cfm?target=Raypak_Digital.htm
I also like Pentair heaters. If you really want to go all out Jandy has a Hi-E2 heater that 95% effcient compare to 82 -84 for others. they are pricey. I don't install the gas so can't comment on the tank size.
Thank you for the info. Would you say the 336 btu is close to the size needed or should I go up to a 400 btu to be safe?
Thanks again,
John