72 duster

slid over from bbodies to abodies. have a72 duster with sbp. bought some cragars 15x6 and 15x7. was going to run 215-70-15 on front, but not sure how large tire I could place on back. front are 3/34 bs and rear are 4/18 bs. stock spring setup with 8 3/4 rearend. any and all help is always appreciated. thanx