I need to purchase a spa heater (120 ft surface area) approximately 2700 gallons.  It heats a jacuzzi in our small place in June Lake (elevation 7,200 ft).  

It needs to be propane as that is the only option in this small town 

We have another Raypak model C-R406A - EP - X ASME that works fine and heats our indoor pool.  It puts out 399,000 BTU's.  The other heater also has the blower (D2?)  

Which model do I choose for the small spa?  How many BTU's do you recommend?  

It goes in a heater vault room that is vented to the outside.