i have a 30 x 40 shop that is not insulated. I have a 1150,000 btu heater in one corner.  when its really cold like 10 to 20 degrees it is a struggle to get some warmth in the shop!!  it basically creates your own wind chill factor!!  last winter while i was rebuilding my front end i bought 2 cheap taps and hung them along one of the trusses so that i was only heating a 12 x 30 area and the heater was great.  I usually only heat my shop to around 55.  any warmer and i might get too comfortable and want to nap!!
In short a 120k heater in 2400 sq is gonna struggle depending on your ceiling height and your outside temp. I think I would go ahead and install it and see how it does, if its not enough during real cold weather you could put another one in another corner.  just plan ahead enough to run a big enough gas line to the building to handle the load!

My shop is two story 24x24 with a single 12x26 add on  with r-12 walls and r-24 ceilings.  The shop has a 18x24 foot set of hot air solar collectors on the southern roof exposure that supplies about 50%of the heat, the other 50% goes to the house.
  As a back up I use a NG fueled 85,000 BTU high boy house style furnace.
I keep the shop at 45 degrees all winter and it takes about ten-fifteen minutes to bring it to a 65 working temperature when I'm out there.

If I had it to do over again I'd have built it waaaay bigger, about 36x40, even that would prolly end up too small.:-)

i have a shop roughly 3000sq ft and 2 super long gas fired heaters just under the ceiling i have 2 they are each about 30ft long.



 

My shop has foam insulation on the ceiling, I have 14ft. walls with a center height of 19ft. I also have insulated doors. Since it was given to me I will hook it up and see how it does. Insulating the walls will have to wait until next yr. So I'll get to see how big a difference foaming the walls will make.