A body Dana 60

So I bought a car that has no drivetrain and need to put one in, so far I have the engine and tranny but I'm having trouble finding a reasonably priced 8 3/4 so I'm thinking of upgrading to a Dana 60

I know a 60 is overkill for a small block but maybe in the future I will want to jump up to a bigblock.

The questions are, how much horsepower do you loose in parasitic drag having to turn the bigger Dana 60 rear diff in comparison to a 8 3/4?

I don't need a 1000 hp rear axle, is there anyone who builds custom lenght basic rear diffs good to 600 hp or so for a reasonable price?