Anything other than the 4.2 will be a custom conversion. I see your options as either a 4.0 or Chev V8 as the simplist. I think the 4.0 would be simpler but others would argue that point. Either way to pay someone to do it will cost a fair bit.
If you aren't already into the rig for a lot(lift, lockers, etc), I would consider selling it and buying a newer rig with a 4.0.