On paper a 383 looks the best.The 400 blocks run hot and are very prone to cracks.I would try and run a 383.With the bore of a 350 and the stroke of the 400 you should have plenty of torque.I would try and run a 6 inch rod.The piston will dwell longer at T.D.C. and you will have better piston skirt life and the rings will last longer.There is also more metal between the bores and you are less likley to blow head gaskets with the 383.As far as the cam try Isky or Howards the both have cams for the lift rule.