If it is a virgin barrel, you could make your gas port location match your parts, although someone posted in another thread that they had to take back the shoulder of the gas block journal area on their barrel a few fractions of an inch in order to fit their gas block properly (not a huge task if you already have someone drilling your gas port and cutting your extractor relief cut).
If you really want an 11" barrel, don't be surprised if you have to be extra thoughtful to get those measurements correct (you would have the FH and gas block very close to one another).
SBRs are cool and all but that thing is going to be a BLASTER.
ETA: to my knowledge, there are only two gas tube/gas piston lengths (three if you count Micro/MAR but that is a whole different animal), and the only difference between R5 and R6 is that the gas block/flash hider are those 2 inches closer together. I could be wrong, but reasonably sure?