If I recall correctly to fit a a v8 double hump transmission cross member on a 70 you have to slot the bolt holes and cut a little out of the outside edge of the center piece of metal. You have to do this because the transmission tunnel is smaller on the 70, very easy to do. I remember reading that you can also take 2 six cylinder cross members and cut them up weld them back together and make a v8 cross member.
You can do what Mike said or you can get one from Frank, "71gold", at least I think they will fit a C-4. you can always ask him.
Measure the distance of the frame holes from the 70 cross member then cut it at appropriate spot to weld piece from the other cross member. I am sure if you have both in your hands you will see what is needed to be done