If I remember, the A500 side connector is 86 pins, 0.100inch pin spacing, 43 pins per side. Haven't looked around for those.
I have just a couple days ago been looking for the 100pin Zorro connector. It's a FoxConn es05006 edge connector. Hard to find these days. Pull them off broken scrap Amigas, or check out
http://www.dfsales.com/mark/displayresults2.asp?PARTNUMBER=ES05006I plan to order some soon from here.
Other ideas I've had in the past, include a genuine A2000 Buster chip. Almost everything needed is on that side connector. Not sure if other things are completely important or not, might be able to bring wires out if they are. Compare A2000 and A500 service manual schematics. Alternative to including an A2000 Buster, use an FPGA and make your own Buster.

Another idea is to make a connector to plug in a Zorro backplane daughtercard from A3000 desktop. (Or A4000 desktop, not sure how different they are beyond the video slot) This lets you minimize the new slingshot design in terms of board size and what you need to do with it, and use an existing, established card for the rest.