I'm no expert, but it looks to me like your schematic calls for a female Packard connector, which you can buy a bunch of for less than $17.
https://www.amazon.com/Packard-Femal...0A8X35QU&psc=1
I'm not sure whether a modern standard 1/4" female connector could substitute for it, but they fit fine in the fuse box and on any aftermarket lamp terminal I've bought so far. with the exception of the pigtail connectors for the turn lights etc, I've yet to have to go beyond the selection of connectors I got at Harbor Freight for just a few bucks. You'll need a crimper/stripper tool to make these connections.
https://www.harborfreight.com/electr...kit-67685.html
https://www.harborfreight.com/electr...ors-66729.html
https://www.harborfreight.com/electr...ing-67598.html
If you have a crimper or solder iron and want to try the standard connector on your ground terminal, I'll drop a couple in the mail for you, they are essentially infinitely cheap.
-Will