AR: Barrel and Handguard/RIS Combo Question

[RacingManiac]

I need some help and insight to this....trying to build a 2nd upper for my G&P WOC, I want to use a 16" outer barrel(from Dytac), which has "Mid-length" Gas system, whereas the original gun has a carbine length system. I want to use a RIS system that is just short of the gas block so that I can use a Vltor style flip-up front sight gasblock. I am not too sure as to what length I should be looking for....

btw if you have a similar setup on your AR it would help a lot...