GM person about tomake switch. 73 dart sport Questions

One thing to look for if you plan on adding power is what size the rearend is. I made the mistake on my first Mopar. Got a 7.25" rearend bought a new motor and started doing research, come to find out I need a new rearend. I understand there are several options here but I am still looking for the best/most effecient. Oh that and the 5 on 4" bolt pattern. Not sure what year they stopped this but that was a shocker when I went to buy wheels. Again I new nothing about Mopar and bought my Dart because I liked it and wanted an old car to play with.