I have a 7500 generator and transfer box. Wouldn't I use a 60 amp power inlet box, since the generator and box is rated at 60 amps

You have to have a power cable that connects to the 240/120 volt outlet on the generator, and then the transfer box has to accept the other end of that cable. So if the far end of the generator cable is made for a 60 amp inlet then this one won't work. But if for some reason the generator outlet is rated at only 30 amp (maybe there are two of them?) and it has a 30 amp power cable then this could be the box you need.