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 fined steel version)
If you want to put it into a stock 340, it will be vary expencive 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: