a.d., you have it right, but 1 ton or 3/4 ton will do. Swap the whole front end to make it easy.
I have seen a couple of the changeovers on the 67-72chevytrucks.com site and I know a guy locally put a 78ish one under his 70. From what I have seen and read, the frames are the same dimensions from early 60's to 87 for 1/2 tons and 91 for one tons(square body only). I believe there are some holes to drill and an adapter to mount the power steering box but there was a guy making and selling those on the 67-72 site above. They have a wider track width so many will also install the later rear as well as it is wider also unless you get a cab and chassis which has a narrower rear closer to the older trucks. Many have put the later rear in regardless because factory gear ratio on 50's and 60's one tons are around a 5.14 ratio and the later rears are usually 4.10 or 4.56 so that makes them much more highway friendly and repair parts are usually easier to find.
