Are you calling the MFG directly? Most MFG don't want to deal with customers, that's why they outsource their products to wholesalers. You'd have better luck calling a supply house who actually sells the products and if you have questions, they will get in contact with a rep and find out for you.
Side note, what sort of building do you need that size UH for?!?! Holy crap. We have a lot of 7500 sq ft plus buildings here heating with 200k units. Old shop used a pair of 300k Reznors to heat a 140x300 warehouse at 42,000 sq ft