Damn too bad found one on CL for almost free. Thanks!
I'll keep saving for the 340 build...
If you are collecting 340 parts anyway then I think the early 340 balancer will work on a 318, the '70/'71 340s had forged crank with internal balance.
Also I think a 318 magnum one will work, but you'd need to get the non-serpentine one to match the LA ancillaries. Some of the 360 Magnums in trucks had non serpentine ones (like on a 360 crate motor) but I'm not sure about the 318s.
I've got a 318 balancer here under my bench but I guess that's not much good 2,000 miles away !
If you find an "unknown" one just look whether it is the same all the way round, if so it will be the correct (neutral) balance, ie. one without any sections cut out.