Definitely buy the wheels and tires first and then have the rear end made to the proper width as mentioned above.
You can always buy the wheels first and then bolt them onto your current rear end to give you a better perspective of what needs to be done to center the wheel inside the wheel wells. Of course have the cars full weight on the suspension when you’re doing the measurements.
A 275/60-15 tire on an 8” wide wheel is about the limit here and you really have to be on your game to stuff them in without rubbing.
This thread may be helpful, 1964 & 1965 are the same when it comes to wheel and tire fitment:
https://forums.maxperformanceinc.com...d.php?t=526735