Can someone help me out Im trying to figure some stuff out...

I am building a custom truck so specific year of bed doesnt really matter. What I am trying to understand is the different sizes and "shapes" of the beds...

My truck is a 36, and I am looking to find a bed similar in this style with the angled upper bed sides, am I correct in my understanding that these angled sides were on beds from the mid 30's till 54? and after that they went to the flat top bed sides till the 70's? These flat top beds were also deeper?

I have come across a bed from what I am told is a 51, price seems fair, needs some work but the upper rolled edges and pockets look to be nice and straight. Problem is no tailgate. How difficult is it going to be to find one. I really like the GMC script logo, and would love to find one of those. What year gates will fit on a bed from a 51 chevy?

Any help/suggestions you guys can provide would be appreciated.