Too narrow - like it doesn't fit in the 512K's motherboard's ROM sockets? Or there's a clearance problem, and something is bumping into something else?

I was looking at this too. With one extra connection to the CPU, I think it would be possible to make the Flash ROM reprogrammable from within a running system. So then you could alter the contents of the ROM disk, or do other experiments, without having to open up the Mac and pull the chips back out. I've got some parts on order to build one of these, then I'll try this to see if I can make it work.