You found a BabyAT case and want to hack it to (u) ATX?

BabyAT cases are much rarer these days than anything ATX (which can essentially be bought new). Please don't mangle it. Instead swap it with another enthousiast for an ATX model.

I wasn't going to modify it at all. The question was looking at the specification I thought it may support both without explicitly stating it. Looking online I also saw the claim that some case makers supported both when atx was becoming the new standard. I was asking for a sanity check if this case may be one of those. 305x244mm seemed to fit. Trying to convert the dimensions to a few specs I could find for screw holes on ATX seemed like it may actually line up. I don't have an ATX board to throw into it. If I really have to I can go and find one I can buy purely to try to screw it in, but I was trying to avoid that.

Some of the screw mount locations between Baby AT and ATX are indeed the same, but all AT cases (supposedly including yours) lack the rectangular I/O back panel cutout that showed up with ATX, and typically aren't tall enough to accommodate the upper 1/3 of an ATX board; hence the sentiment about needing to mangle an AT case to make it work for ATX.

If only custom backplates with AT keyboard, LPT, 2 COMs, 2 USBs and PS/2 connector were available.

That makes sense. Height in this instance doesn't seem to be an issue - there is just under 7 inches of clearance to work with. I hadn't thought about the backplate at all. The entire expansion slot area is removable and separate from the case. However I'm guessing its not an easy swap with an atx case, and even then I may be better off using a baby at here and trying to find a rack mount atx case.

Thanks