Most uppers are gonna fit your S&W lower just fine. I have one and it's worked with every upper I've tried, including Colt, ArmaLite, Daniel Defense and Bushmaster.
If you run across one that doesn't fit, it's not the fault of the lower, S&W makes them to proper spec.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek