It will bolt up, but the frame horns are different. The 74 has cushions between the bumper and frame, which the does not have. If the subframe on the 70 is solid, you would be better off just swapping the disk brake set-up to it.
If you just swap the brake stuff over it could be done in a couple of hours. I think the brake hoses and the routing parts/brackets for the lines on the sub frame and A-Arms are different also. It would be a good time to add the better brake hoses while you are at it.
I believe all of the control arms/brakes etc are the same with 68-74 X bodies and the same as 67-69 F bodies. They all interchange with each other because the subframe is the same. In fact I think the discs also interchange with A body GM cars as well.
The subframes are the same as well, except as noted with the 73/74 front cushions because of the Federal Bumper requirements that was new for 73. I would agree to keep the 70 sub frame since you would have to cut off the cushions on the 74 to make it work. Might be worth it though if the 70 subframe is crusty or bent.