318 running rough and losing vacuum when in gear.

the 318 was just put in last year with supposable 77000 miles on it. long story behind this car. my sister had it, was a 340 but brother inlaw spin a crank bearing. he kept the 340 for another project. so the story goes, he say's the 318 has the 340 flex plate and torque converter on it. I have wondered if that makes the engine unbalanced because it feels like the driveshaft vibrated over 60mph. hence the tranny leak. I have been mulling the fact that the torque converter may be the entire issue with the engine also.