[Arran]

Peter;
I think that I would have been more inclined just to make a 4 pin to octal adapter out of the base of the 5Z3 tube, we have an odd rectifier tube/kenotron that Rogers made here called a 2X3, and it is half of a 5Y3, both in pinout, and electrically, so you need to make adapters, or change sockets for those. They used pairs of octal sockets but Rogers removed the contacts from the sockets that were not needed for the 2X3s, so you can't put a 5Y3 or 5Y4 in there just by rewiring one of the sockets. I'm surprised that the C.G.E A-87 used a 5Z3, a type #80 or 5Z4 would have been more common then, a 5Z3 is like a four pin 5U4, which was normally used in larger sets with push pull output tubes.
That type of paper capacitor, with the rolled over ends, is one of the easier types to restuff, if original appearance is important. You can get the capacitor out by placing a mechanic's socket, of appropriate size on the bench, place one end of the capacitor on top of the socket, grasp the body of the paper tube, press down over the socket, and the inner part will pop out of the top.
Regards
Arran