In a 70 series the 215, 225 and 235 are the way to go. If you go with a 245 or bigger, then you need to step down to a 60 series.
I run 215s on the front of my 64 wagon with 235s in the back, works good. if you want the same size on all 4 then i would stick to a 215/70/15 or a 205/75/15
My winter car is running 15x6 all around with 205/75R15s all around
Depends on how you want the car to sit and how much wheel well you want filled. Might want to find some old tires and play with it a bit. By chance do you have another car you could pop some off to try? Think I tried 4 or 5 sizes on mine.
This may not be of any benefit to you as you mentioned that you are going with 70's but... here's a pic of 15" 235 60's on the front and 255 60's on the rear:
That looks pretty good! You have all 4 tires on 15x8" rally's? Or did you go with 15x7" up front? Trim rings look the same front and back so I'm guessing 15x8" all around?
Think I tried 4 or 5 sizes on mine.