Which rear speakers for a 67 Valiant?

Just about any 6 x 9 speaker will fit, just don't use the grille that comes with them. The package shelf cover will provide the grille function. You will need to remove the back seat and slide package shelf panel cover out. This will expose the metal structure underneath. Install speakers from the trunk. Check underside of the shelf panel cover to make sure that you can see through the cut-outs. It's OK if they're covered with gauze like stuff.

Put a thin layer of foam (¼ inch max.) over the package shelf with speakers installed. Make sure the foam is cut-out to clear the speakers. The purpose of the foam is to prevent the package shelf cover from reverberating (rattling) when your tunes are on. Slide the cover back into place.

If you have vibration issues in spite of precautions, install a pair of upholstery buttons through the shelf and cover. You'll need to paint or cover the buttons to match the shelf cover. I have used cotter pins to secure the buttons which make them both removeable and adjustable for tension.

The electrical harness runs down the drivers side of the interior. Recommend running equal length of identical wire down the passenger side from your source to the speakers. There is a little protected area in the lower part of the floor pan that is an ideal place to run the wires. It requires removing the passenger door sill and pulling back the carpet to reach. Once wires are run, put the floor back in order, then re-install rear seat.