misfire on acceleration.

i have a 1990 f150 larait 300 6cyl. i recently did a tune up and got cyl 4 and 2 reversed on the cap.. i feel dumb but now it runs alright at idle. took it for a test drive and i made it about a block and it started to misfire badly and stumble. it would accelerate but not at the rate it should. i think i need to adjust timing? also there is an intake gasket leak and a friend told me that the leak may be causing it because there is too much air getting into the cylinders. any possibility that could cause it? i appreicate any help because this is stumping me. the fuel pump is only a couple years old. i just replaced the fuel filter.