The motor mounts wont bolt on as there are no bolt bosses on the 389 for an F Body car.
Also the 64 and earlier 389 had no bolt bosses on the block for a starter either.
You need to get a 70 or later block as they have 5 bolt bosses for motor mounts. Even then some block may not have the bosses drilled and tapped, but that isnt hard to do when you are building the engine.
For a good street engine the 455 is a good choice. No replacement for displacement.
Before you buy ANY block, do some research and ask questions.
Dont get that 1963 389 you are talking about, unless you want loads of dissapointment.