As far as I know, the only case that will accommodate the IBM PC's motherboard properly is the one in which it came or a slavish clone system. The XT and XT/286, on the other hand can be shoehorned into a generic AT case with little difficulty, but the AT boards are beasts and require a beastly large case to match. The PC has five slots and two DIN sockets while the XT and AT have eight slots and one DIN socket. The spacing of the slots is different between the PC and XT & AT, and the position of the keyboard DIN is closer to the power connector on the XT and AT. If you put a PC motherboard into an AT case, a good portion of the keyboard DIN would be blocked, the cassette DIN would be totally blocked, and the expansion slot cutouts on the back of the case would not match the positioning for most, if not all the PC's motherboard slots. This must make for difficulty unless you have a metal cutting tool and probably welding equipment if you are trying to make a PC motherboard properly fit into an AT case.