Eskins,
Things you should know...
Most 35Us were on a WorkHorse Chassis and were either a Ford 6.8 or a GM 8.1. As both are gas engines, any good starting battery that fits in the space will work for you. Look at the battery. It will have a number on the top like 27, 27F, 34-72 or the something, write that number in the owner's manual. Just match the number.
Before you do that, is the battery bad? Charge it, let it sit for a day. If it cranks the engine, it is good. If it seems bad, clean the terminals and try again.
Matt