I think before you can determine where to cut the shaft, you'll need to know exactly where the box is going to mount. If you're not going to buy a kit, perhaps you can obtain a pattern of the adapter from someone. It's critical to locate the box in the correct position and steering shaft angle or the pitman arm will be out of sync with the steering linkage.
Assuming you either buy the adapter plate or can fab one up in the correct dimensions, there's another problem with the 1960-62 frame. The available conversions are made for the 63-66 frame, which is taller in the area of the steering box, and is also a single flat piece of steel where the plate/box bolts on. As shown in photo 12 on the 2nd link above, the upper rear hole lands in the flat area of the frame. On a 1960 that hole is directly centered over the top flange of the frame. The only way to use that hole without modification is to cut a slot across the top of the frame to allow the bolt to reach the other side.
Rather than cutting the frame on mine, I drilled another hole lower on the plate, and a matching hole lower on the steering box (pic attached):
This way the top of the frame remains undamaged. The downside - after 10 years my original p/s box died and required replacement. I drilled the new one, no problem - except - the core cannot be turned in for credit because of the extra hole. Other than that, I've put about 165,000 miles on the odometer with this setup and it works fine. Unless you have superior fab skills I suggest buying an adapter plate at least. Captainfab sells a version on this site. I bought mine from REZ (they were the originator and only company making them back when I did the swap).