Can I replace the eccentric bottom bracket in my Salsa El Mariachi with standard BB?
I've never seen something like that, an eccentric to concentric conversion BB. Even if it existed it would have all the issues of an eccentric, so the only benefit would be if you really wanted that otherwise unattainable chainstay length. If you really, really wanted this, just having one made would be pretty easy. If you're having creak troubles you could try a Niner eccentric, which has been known to solve the problem.