Stare at it long enough and it will come to you.
I'm doing the cab to frame mounting a little different than most people because. This is how I'm doing it on the ones that have bad floors at the cab mount area, which is most. The AD cabs have very little bracing where the cab mounts land. Later years had a raised, upside down, "hat" section for the mounts. The neat part is that this allows reusing the S10 cab stands, although they do get relocated.
What will I do when I get one with a nice floor? Probably not do the frame swap. Either restore it to original (or original with upgrades) or do the subframe swap so that I don't have to even mess with cab mounts. Here's a wireframe drawing layered on top of a cab pic to show my method. In reality, it will probably end up shorter than that.
Junk cab to play with
Most people are building tall cab stands to reach up to the original floor but Frame Twist.
For the rear, this hat section is typically shorter in height, around 2 1/2", and runs the full width of the cab floor. The stock hat section on the 47-54 AD trucks is less than 1" tall. Some of them also had only one rubber mount in the middle of the rear cab and two at the front of the cab. In the old days, truck frames would flex for the entire length and so they designed for that. On the newer trucks, like the S-10, the frame flex is mostly from the cab back because they box the frame from the front tips to half way under the cab.