i got an 86 f250 4x4 manuel 4 speed overdrive with a 302 with a very bad backmain seal, had it replaced and if anything it's worse. was thinking of just replacing the motor because the 302 is really lacking for power to pull my 20' gooseneck stock trailer. whats the biggest motor i can put in without having to change everything. 351? open to suggestions. would really like to have a diesel dually but figure i'll probably just get buy with this for a couple more years.