Hmmm, how about shopping around for another '68 cab to go with the one you have and build a crew cab. A front-wheel drive Toronado or Eldorado power unit mounted up front would made the drivetrain easier. With a little scrounging, you could make it with all '68 GM parts. Use the front-drive core support, radiator, fan, shroud and all AC parts. Use the T or E rear axle under the rear so that all the brakes and valves match up.