How do you replace the blower motor? The blower motor is not visible on a 75000 btu furnace .
There are going to be screws holding in what is commonly referred to as the squirrel cage, the squirrell cage being where the blower wheel and motor are. If it is what I'm thinking it is than you remove those screws and the entire squirrel cage assembly slides out on it's own rails, those rails having gaurds/guides from the furnace that they slide in and out of.
Sometimes tricky getting it back in.
Take some pics and I can better assist you versus guessing as to if this is the kind of assembly you have.