Swap from 7.25 to 8.75 advice

So my son picked up a '71 dart parts car. It has the 8.75 in it with 2.76 gears, 741 case. He bought it to swap the rearend into his '71 valiant to upgrade from the 7.25. His question was, "do I have to swap to the springs that came with the 8.75 or can I reuse the current springs from the 7.25." I told him I thought that he would need to swap the springs out as he has to use the 8.75 shock plates with the 8.75. And it seems to me that the 8.75 would have thicker stiffer springs and his 7.25 springs are sagging anyway. I think his reason for not swapping them out is the extra work to completely remove the ones in his valiant.