The S&A is a good unit. Some fitting is required more often than not. Sometimes the S&A MSH won't fully install over the sear spring, so filing a clearance for the sear spring is something I find myself doing a lot.
Sometimes the MSH hole won't line up exactly with the frame holes, so I use a round file to slightly enlarge the MSH hole to get enough clearance to install the MSH pin. If the hole is really off, I'd suggest returning it and trying another brand of magwell.
And sometimes there is a noticeable gap between the bottom of the frame and the magwell. There isn't much that can really be done about this by the typical owner, except for installing some longish magwell cut grips that hide the gap. This is where a two-piece design with a detachable magwell can come in handy, since the MSH can be filed down enough to close the gap.