RE: 340 or 440-6
My suggestion is to keep the 340 in the Challenger and put the 440+6 in the GTX. The 78 440 is a motorhome motor, it is thin walled compared to the thicker walled 440's that were built 73 and before. The 340 will rev higher than the 440 will, even though the 440 puts out more torque off the line. Unless you rebuilt the 78 440, it has a cast crank and not a forged crank like the 70 440+6. For power purposes, forged is way better than cast.