If all you want to do is a disc swap, you don't need control arms, or a crossmember change. The disc swap kits sold by many aftermarket companies will work on your 70's factory control arms. You may need to change spindles, depending on the kit you buy. Just make sure you buy the kit for your truck. If it's all factory, you just need to make sure you look at a swap meant for your 70. If you or someone else had swapped to a newer control arm, spindle and/or crossmemeber from a 71 up C10 then you should already have discs on it.
The option Jim suggested will work if you're looking for a factory type setup or want to avoid aftermarket parts.
I see you have asked this question again in another post. Refer to that thread for more info...
http://67-72chevytrucks.com/vboard/s...d.php?t=684559