will a 93 8ft fit on a 83?

the one on my 83 is rusted out pretty bad.

 

So long as they are both long box's (the 8 foot bed) they should swap over fine. I think filler hoses are the only possible difference. The biggest difference is the wheel wells will not match front to back then. Somewhere on here there is a thread about doing this exact swap and it works.

 

We see this pretty frequently in my area (northern New England) so it does work to some extent. Another difference you may run into is that Ford went from using 8 bolts to secure the bed to 6. I think that was at some point in the 1980s. Just mentioning this for the sake of completeness.

HTH.