Would 98 chevy bed fit on a 90 Chevy truck?
Sure, as long as they are the same lengths, long box, short box. The only thing I can for-see is maybe the bolt holes not lining up, but just would have to mark some and drill out new ones. Also may have to shim a little. Otherwise they are the same size.
The real question should be - is it the same body style? According to Wikipedia those two years are in the same series trucks so the bodies are probably pretty much the same. https://en.wikipedia.org/wiki/Chevrolet_C/K#Fourth_generation_198 8.E2.80.932002_.28GMT400.29
A good visual comparison is in order.
Looking for a Used C/K 2500 in your area?
CarGurus has 214 nationwide C/K 2500 listings starting at $2,250.
Search Chevrolet C/K 2500 Questions
Chevrolet C/K 2500 Experts