so I guess i'm looking for a 340 for my 69 dart

340 was 68~73 early were forged crank and internally balanced, later were cast crank and externally balanced.

why not go 360? cheaper and easier to source, same price to build.

you'll need motor swap mounts to keep your slant six k-member (subframe), or you can swap the K to a V8 one and use stock 340 mounts.

here's the link for that: '67-72 A-Body Slant 6 to 273/318 or 340/360

you'll also need a whole mess of other stuff: transmission, radiator, some light wiring and obviously exhaust. basically none of the slant stuff crosses over.