If you haven't, invest in a set of Ergon grips with bar ends for your FX. Don't know your stock tires, but slick 28's or 25's both worked well on my Sirrus, and I put many 75 to 100 mile rides on it in good comfort. Don't know what your stock bars were, but I have had great luck with flat bars, 5 degree rise in one plane, cut down fairly narrow.
This whole "flat bars over 35 miles are uncomfortable" thing is a big pile of possum poo. If the bike fits and you really like it, spend some time and a few bucks to tweak.