Only if your '86 came w/a 7' bed to start with. The 7' box wont fit the a 6' box frame w/o modifications. The 7' beds are longer between the cab and wheel well than the 6' beds are. You would have to section about a foot out of the front part of the 2x4 bed to get it to fit.

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