What Are You Doing To/With Your Car Today?
Maybe you can squeeze the 15 x 8s in there without moving the spring with the proper backspace rims.
4 1/4" and 4 1/2" backspace seems pretty common on the 67 up Darts. Center the rear end as they can be off from side to side by 1/4".
Gonna be tight, but sure some of the guys on here will know what works and what does not work.
What tire are you going to want to run ??