In addition to the bed mounts not lining up and the frame needing to be lengtheded, the wheel well would also be in the wrong place.

For the short term, you might consider building a wedge shaped box that the front tires of the bikes mount to w/the riding gear underneath.

Depending on your fabrication skills, you might consider building a flatbed designed to hold the bikes, built in storage for riding/camping gear, and a pull out ramp for loading the bikes.

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