For the 2nd gen and 3rd gen trucks the IFS frames are the same.  So are the cab mount locations on the cabs.  The solid axle trucks have different cab mount locations, so an IFS cab will not just bolt right up.  It is fairly easy to move the cab mounts though.  All you have to do is to take off the old cab, leave the front mounts attached, but cut off the other cab mounts CAREFULLY and SAVE THEM!!.  Set the newer cab on, slide the bolts inplace for the two very front cab mounts just to hold the cab in place, so don't tighten them down ( Slight redrilling may be nescessary)  Then mock up the other cab mounts that you had cut off by bolting them to the cab where they go and then using some bolcks, like scrap 2x4s or something to get the cab held up where you want it, then just tack weld the cab mounts in their new location.  If you just tack weld them on you can adjust them if you need to!!  Then you can just lift the back of the cab up enough to solidly weld them when you have them where you want them.  I have done two cab swaps like that this year.  On the ones I did, I moved the cab back about 1.5" or 2" from it's origional location to clearance the front firewalls with the tires better.  If you do that though, you need to think about the steering shaft length, I happened to have a couple of 2" shaft spacers from an old body lift kits that I used to lengthen the steering shaft.  Hope this helps!  if you have any questions I will try to help.