I would think that would be plenty of heat as long as the building is at least decently insulated. Apples and soccer balls here but I have a 120k btu modulating boiler in my 54x72x18 and it rarely goes above one of the first few heat levels. I’m in Central MN and am very well insulated.
As far as tube heaters goes ….. I have 28 20’ LB White 80k btu tube heaters in my chicken barn that is 60x624x10 and is insulated rather cheaply and they’ll heat it up from 45f to 95f in around an hour. Tube heaters are great.