James a lot of states if not all used the engine number as the vin number on the title on the trucks back when these trucks were new. I'm not sure what the theory behind it was but that is what they did then.
When I bought my truck in Texas in 1973 it had an "engine swap" notation on the title an used the body number as the vin.
If you are building from scratch starting with the cab and plan to use a stock frame I'd hunt for a complete truck with a title with a good frame and bad cab that you could get for a reasonable price.
If you are planning on putting it on an S-10 frame I wouldn't be that excited about the number plate as long as you have a good bill of sale for the cab.
I probably shouldn't mention it but over the past 30 years I've bought more then one really trashed AD truck just because it had a clear title and the proper id plate. One we honestly carried home in the back of my 48 a few pieces at a time making several trips. The owner had taken it all apart and then lost interest in it.
Here is a link to a site that lists the differences between cabs. Actually more than one.
http://53truck.ez15loan.com/howto/years.htm
http://www.adchevy.com/info/yeartoyearchanges/
http://www.pickuptrucks.com/html/his..._segment5.html
Hope that helps.