The only difference is most 318's are cast cranks except the early ones and some of the trucks. Unlike the 360 cast crank though, the 318 cast cranks are internally balanced so they will work in the 340's if you don't mind running a cast crank.
They will both fit(318-340) but the balance is different so it would have to be balance.........this could be good or bad(Talking about the hard to find steel version)
If you want to put it into a stock 340, it will require Mallory metal, which is vary expensive
Now if you were looking to make a high winding 340 W/ light weight part,(aka Light Weight custom Piston and alu. Rods) the 318 crank could cost a lot less, to balance.
I have a 318 steel crank in a p.o.s. dodge motor home, that i had thoughts about............But have decided that a slow turning Stroker will be much more maintenance free, and longer life then a zinging 8000+ rpm 340 and cost less too!........But it still would be fun:cheers: