Why did Carlo Gambino pass over his longtime Underboss, Neil Dellacroce, in favor of Paul Castellano? Besides their relation as being cousins and brother in laws, was there any actual merit that Gambino considered?

Also, before Carlo died, did Paul and Neil BOTH serve as Underboss at the same time? I've read some charts list them as joint Underbosses in the '70s, while others list Neil as Underboss and Paul as a Capo.