1971 Demon Rear end upgrade

Looking for advice on upgrading the rear end on my Demon, thinking 9.25 with disc conversion. Also wondering what the gear ratio may be in the existing stock one.
Suggestions on a make?
Where to buy?
How to properly measure existing?
Let the Horsepower begin!!
I went with the 9.75 Strange 60 sold by Dr Diff. Just about a bolt in swap from the 8.75.