So what happens if you keep the old /6 rear end after 360 swap?

Bought a 74 Duster with a 318/904/7.25 last week and it came with a tore up rear with Big bolt. Went to the field and got another from a 4 dr dart donor and cleaned it up and installed it. I think the issue and the longevity of the unit is very closely related to ones ability to use it as a driver and not a racer. It might last a granny for many years but i'm sure you aren't a granny. The Duster i bought with the bad rear did come with 12" tires on the rear with a 7 1/4 in the back. I wonder what happened? Duh.... The small unit will last you until you have the ability to afford the change you want but you will have to keep your foot out of it and run not so big tires.