Sounds like you have a plug wire on the wrong plug. Check your firing order again.
As well, if you find they are all in the right order, pop off the dist cap, check the rotor gap (as cp suggested earlier), check terminal condition, and all wires seated fully after the cap in put back in place.
I had a persistent backfire two years ago that I eventually traced to a cracked #3 cyl header tube very close to the head. So check that your headers are sealing well against the head and that none of your plug boots have burned being too close to a tube.