Yea I was just wondering if a 68 model 318 steel crank will work in my 70 model 340? We found out or crank was cracked so we were going to put the crank out of the 318 in it if it will work???
The 318 steel crank is a direct size replacement but you will find the counterweights are a bit smaller/lighter, so unless you go with lighter 340 pistons the 318 crank will require mallory metal to be added to get a proper neutral balance - this can be a bit expensive but worth it in the end - IMHO.
You could use a 360 cast crank - cut down the mains to 340 size and have a ±372ci 340!!!!