Looking at this layout
download/file.php?id=724&mode=viewSee how that lug that 5 wires in it? See how that lug has two wires going diagonally from it towards two other lugs? Both of those lugs are ground lugs. You can run your ground wire from either of those lugs.
Also you don't have to put wires right through the lug. Feel free to wrap the wire around the lug. Just remember to make a good solid connection whatever you do.
You should be able to find many tutorials on the forum and online about wiring inputs, especially with Cliff Jacks.
I'm with you though, co-ax and wiring inputs is a pain!
Antique Electronic Supply (Tube's and More) should sell co-ax.