I've both a cast and steel crank for a 318. $75.00 and $100.00 respectively for one that will need to be ground. UPS on top of that. If your engine is still good, complete, running and you just need to mate it to a '69 904 converter, I'd just have a converter built for it. My local builder does them for about $75.00. If your motor is apart, then sure, go ahead and change the crank but you should do a full engine balance to do it right.