The tail shaft length is different on the transmissions that mate to a 203 versus Dodges other tcases. Some people have swapped in a later tcase without swapping trans or tail shafts, but I think the spline engagement length is less than desirable IIRC. The best thing to do would be keep the trans and 203 and mate a 205 to it in a doubler. The (probably) cheaper option would be to swap in a trans and tcase from an 80s dodge with either a 205 for the strength or a 208 or 241 for the lower ratio. Head over to ramchargercentral.com and look around. Tons of info over there for these era trucks.
Duane