318 crank into 340? pitfalls

building a 340 that had very lightend pistons,rods,pins.crank was balanced to this.rods would have to go to .040 over which I don,t like and my mechine shop told me we could go to a 318 crank and rods to make balancing easier,taking weight off instead of adding a lot of it back in.located a 318 that's all there for $75.00.is there any problems with doing this.all the guy could tell me about the 318 is that it was a mid 70's and in really bad shape.what about pillet bushing,will it be drilled? or did some come with and some without?what about harmonic balancer?and the flywheel?any help would be appreciated.