I have two S&W MP15 sport ARs.  Planning to mod one to remove the A2 sight and install free float rail.
This brings up several choices such as going with carbine length guard and rail height gas block or, going with
a mid length rail over a low profile gas block since gas port on barrel is always going to be carbine length.
This is further complicated by questions like alloy or steel gas block.  I am sure readers get the point.
Just interested in tapping the collective wisdom of the more experienced members.

Mr Mac

With the plan to use a free-float rail, look for a rail length of 9-12" inches. The advantage is being able to extended your support hand out, lessening the bend in your elbow, to find your "natural point of aim". If a flashlight will be added on, the closer the lens can be to the muzzle, will reduce the amount of shadow the barrel throws due to blocking part of the light emitted. For a low-profile / micro gas block, I would look for one with a black nitride/Melonite finish due to its corrosion resistance, and resistance to accumulating fouling. A lot of the low-pro / micro gas blocks are chromoly steel (i.e., 4140).

You forgot one option, and that is cutting down your current FSB to fit under a free float rail.  This is probably the best as well as the cheapest way to go...

I was just about to throw this one out there Hahahaha  +1 on above.