If your actual axle ratio doesn't match what the car originally came with I can almost GUARANTEE the original failed and was replaced with a used 3rd member at some point.
Those 7 1/4's are weak at best. Our '66 Cuda had 4 between 1966 and 1982 and it is an auto.
Yours is a 4 speed I see?
Guarantee it broke a few too over the years.
Not saying I would replace it immediately or anything, but the first time it needed anything besides 90W I would plan on an upgrade to something else.
A lot of that depends on YOUR driving habits of course!!
