My dog has ridden with me a lot over the years. She is a 45 lb cattle dog and might have better balance than a 70 Pit, but I hardly notice her on back. 70 lbs would be more noticeable, but both of you would have to get use to it.
I have the Cyclerack and put a piece of foam covered by carpet and all secured with a small cargo bungee. The carpet helps her with grip.
This is probably the best photo showing her and the pad.
Here I made an extension to the rear of the Cyclerack. It did make more room, but with more weight rearward, it made the front end a little lighter than I liked. You'll have to experiment and
see what works. You may have to make a platform a little larger than mine with a dog your size.