The fit would be better between the same gen X-cab and stand. cab, than between 2nd and 3rd gens as in the other post. The 3rd gen X-cab's are a little taller and I believe the back slopes a little more than on the stand. cab. I would try the shell(if the owner doesn't mind) and see what you think.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed