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.
Going from memory here but I think this is the furnace that you have to remove the exhaust vent pipe as well to get to the squirrel cage. This entails removing the pipe from the furnace and once the service door of the furnace is off you have to remove the cover plate of the remaining internal vent pipe which runs horizontally (and directly in front of the squirrel cage) for about 14" to 16". You have to then remove that piece of vent pipe as well. All you really have to look for are the screws that hold things in your way and remember how it goes back, which is not all too terribley complicated.
If this is that kind of furnace than it sounds worse than it actually is. Ten minutes and it's out.