The bed looks incredible ,nice job.
As far as the motor goes I personally would go with the SBC and just put a 700R4 behind it.
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'58 SWB Fleetside...Front & Rear Porterbuilt Dropmember, 6.0 LS /4L60E. Accuair E-level, 20 Billet Specialties (Vintecs)
'63 Buick Riviera ... AccuAir E-level, 20" Billet Specialties (Vintecs)
http://67-72chevytrucks.com/vboard/s...=528853&page=5