Challenger wheelbase is 3 inches longer, IIRC...google it or check Wikipedia for confirmation. As far as the Barracuda/Cuda grilles, '70 has the turn signals in the upper grille area and '72-74 has them under the bumper, a detail that MPC forgot on their kits.. and '70 and '72-74 have 2 headlights, while the '71 has 4 headlights and the turn signals set into the pan.
I have resin '70 conversion parts (grilles, taillight panel, etc) to use w/ the once-common '80s issue MPC '74s, but never got around to building them...also have the MPC '70-74 annuals, 6 of '80s issue '74s, the '74 'Pro Street' version, a couple of the '74 snap kits, the Monogram '70 & '71 kits, but am still looking forward to getting 2 or 3 of the new kit...