318 crank into 340? pitfalls

speed pro l-2316f40 is the one we are considering at this point.not sure yet.machinist has another option he will check on after cristmas.so you would not be concerned about the crank having to be turned to .040 on rods? what is imbalance? machinest said he can ad wight to my 340 crank,just thought it might save me some $$$ to go with the 318 crank.but if he has to add weight to that crank I may not come out ahead.