Get a look at Boston's defense and finish up Alphabet. That will let you know whether you can take Boston now, need to heal for a few turns, or it's outright undoable without major reinforcements; it will also let you know what techs you could extort out of Lincoln in exchange for 10 turns of peace.
My offhand guess would be that you can probably blitz Boston, peace for 10 turns with Lincoln while you take the barb city, heal up, and reposition, then finish him off and I suspect the timing would work out about right. But I could be wrong about that.
Once Lincoln is gone you're going to need to decide whether you want to just keep the war machine rolling, or shift back to a more peaceful footing. You'll definitely have enough good cities that either one should be a pretty smooth path to victory. If you aren't planning on carrying on to hit Shaka after Lincoln, let your current round of horse archer builds finish but shift to stuff like workers, granaries, libraries, maybe more settlers after that.
Lincoln's double corn + copper capital is a very nice prize of war 
. Once Alphabet finished, you'll probably get IW through trade or extort it from Lincoln. As coanda suggested, both war and peaceful development are possible options after Lincoln is gone. You already have maths and HRB, so you only need masonry-construction to unlock war elephants if you want to keep warring; if you prefer to stay at peace, your tech situation is also quite close to currency-CoL.
