Just in case anyone else is doing cartridge PCB layout-- the dimensions for the cardedge goldfingers shown here:
...are not what I'm seeing. Judging by the MP3 cart that I'm measuring, I'm getting a finger length of 5.25mm. (That's quite a lot less than the 6.7mm on the wiki.) Furthermore, those fingers still protrude at least 1.25mm beyond the cartridge connector on the JB PCB. All told, you should be able to use ~4mm fingers with a 0.75mm chamfer and be just fine. While the longer fingers won't really "hurt" anything, the shorter lengths make routing a lot easier if you're trying to use the original cart's screw bosses.
Connector pitch, alignment and overall width matches what I'm measuring with the wiki.