Rear differential suggestions

[Missyblue]

I'm need of advice I'm drowning in articles and build threads haha new to this world and learning slowly......
I have a 56 planning to use a 350v8 from a donor vehicle and 700r4 tranny want to go automatic and looking for daily driver with preference being on better gas mileage than hot rod offf the line. Max speeds usually around 60 mph. Hoping to find a donor differential that won't need too much modification due to its width or lugs I have seen the lists of some that fit but hoping for advice on things that work well and gear ratio arguments. 1 mechanic I talked to recommended 3.73 but curious. We prefer to stay with chevy/gmc but woukd consider others. I know Ford 9" are recommended and someone said dodge dakots.....
Hoping to start searching and watching. Not in a huge hurry as I'm in savings mode and need to find a good deal
Thanks for the advice this place is awesome