I'm working on my 500 as well, and while an 040 in the 500 would be really damn neat, only 4MB, in ZIP RAM? Pass. I'll take my 50MHz 030 with 32MB SIMM, thank you.
I agree with MrA500 on this one: no IDE, etc. By the time I get done stacking the AdIDE and clock port adapter... no room for a keyboard. Although, I do like the idea of Kickstart on the accelerator -- that's a nice touch.
I wonder if I'd get hanged for hacking a SIMM module into such a rare rare rare item... :-)
EDIT: I wonder how this would fare in an A1000.... hmmm....