help !!!!

ok here it goes, i started out with a 1974 valiant which i later found out that the whole roof skin was rotten, seing as though i wanted to change it to a dart style front and rear end i recently picked up a 72 dart which looked reasonable on the outside, as i was loading it on the trailer i discovered the whole front frame rails and tortion arm cross member is completely rotten, the place where i got the dart from has a 71 shell which i have to look into further to see if it is worth while picking up. i am pondering the thought of changing the entire frame rail assembly off the 74 onto the 72, but if i go with the 71 will the disk breaks off the 74 bolt right onto the 71 or 72 ( i realize i have to change to the power booster) and the other question is where can i get a rear disk break axle for a reasonable price. i am trying to get all the parts together for my winter project.

thanks in advance

shane