New Guy- K20 vs. K10

[Growdon]

I am new to this board. I have been a fan of these rigs since I was a little kid. I owned a 72 K5 CST up until about a month ago. It hurts , but I needed to get something more practical and needed a little cash. I am looking to build a flat bed 4X4 with a mild lift, heavy drivetrain (14bolt/Dana 60?) and 35's or their metric equivalent for higher ply tiers. My question is basically what is the difference between a 3/4 ton and a 1/2 ton (K10-K20)? Is there any difference in the frames? What axles do the K20's usually come with from the factory (1971-1972) I know that I am looking for a truck with an NP205, but what 4 speeds came in these trucks, and should I be looking for a certain tranny (my blazer had a TH350).

I have 4 trucks I am going to go look at on Sunday three K20's and one K10.

Thank you.