My 64 econo truck has a 9 inch rear in it and it came stock. I also have recently aquired a pre 65 van body will a 9inch rear also.
You consider seeing if you could reinstall a standard back in. That would probally help. It maybe difficult to find the older trans,but they are a round. At 35 mph it seems like you are not getting out of 2nd. Is the tranny working up to snuff. If so it might be that you will have to bite a bullet and change the gears to get your speed up.