I'm considering making the MP15x into a free floating barrel and adding a longer rail. Any insights or suggestions on how to do this?

Your 2 options are to swap your front sight block with a low profile gas block and fit a free float tube over it or shave down your front sight block and fit a free float tube over it. Depending on what rail you go with, you may need to swap out your barrel nut. Plenty of youtube videos on this.

Thanks for the help! Do you know of any complaints with the set screws working loose in the low pro gas block?

Yes, people have commented previously about set screws loosening and pinned being preferred and more solid.

Take that for what it is worth though, since you have no idea if the ones that came loose were torqued down with enough force, or if thread lock was used...but I still prefer pinned.

I free floated my M&P 15OR using a Daniel Defense Omega 7.0 rail. I went that route since I didn't have to change or remove anything to accommodate the install.

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Stu

Daniel Defense, as well as a couple other makers do make 2 piece free floats with FSB cut outs. The DD ones are high dollar though. That is another route you could look into though that doesn't require any modifications.

I used a 556 tactical (Samson Evolution) 12.37" rail. I removed the standard A2 front sight block and replaced it with a Yankee Hill low profile gas block. The hand rail slipped right over it and uses the stock barrel nut with a thermal bushing (included in the kit) to lock the rail in place. The kit also comes with 4 pieces of standard rail sections to mount things to. I love the rail. Here is a pic of my rifle:
M&P 15 2.jpg

Any chance this rail would clear a "shaved" A2 front sight block?

I'm pretty sure it would. I have seen this on other rifles that have a shaved front sight block. Any reason not to just get a low profile gas block? It's really easy to install.