You probably already know this, but here's some info on the crank just in case. All 68-72 340 cranks were forged. 73 340 cranks were not forged, but were shot peened for extra strength- not quite as strong as forged, but stronger than cast. Also, the 273 cranks were supposedly forged 64-66. There were also some 318s that came in trucks with forged cranks.
The way to tell by looking, besides knowing the casting numbers (which I do not) is to look at the casting or forging flash on the throws. On a cast crank, the casting flash is 1/4" wide; forged cranks have a 3/8" to 1/2" flash on the throws.
It might be hard to find an original forged crank, but you should be able to find an aftermarket forged 340 crank pretty easily. They have them at Summit, Jegs, Mancini, ebay, etc. for around $750.
