[jocko]

Here are the possibilities for this casting number (that I could find):

3963512 427 68-69 Passenger 385,425 HP
3963512 427 69 Chevelle, Camaro COPO (Central Office Production Order)
3963512 427 69 Corvette Late 69
3963512 454 70 Corvette, Passenger 360,390 HP
3963512 454 70 Chevelle LS-6 450 HP
3963512 454 70-71 Pass,Chevelle, Monte Carlo
3963512 454 71 Corvette 365,425 HP

So, if you're sure it's a 68/9 passenger car motor - then you know it's either a 385 or 425 hp eng. I'd say regardless of vintage - any 427 is worth a good bit of dough. The question really should be "why isn't it running"? Is it siezed? Does the seller say what's wrong with it? I'd sure want to know that it can rotate - at a minimum. If it can spin (even by hand with a wrench on the crank bolt) I'd say it's probably worth buying as long as there's no rods hanging out of the side of the block.... as for "how much" - not sure - I'd say a complete long block (that isn't running) may be worth up to $200-$400. If it's in really good shape with all the accessories still dangling (edit-sorry, re-read your post and see it has no brackets, so probably not worth much more), probably worth a little more. If you really need/want it, and can accept knowing it probably needs a rebuild, then it is up to you and your wallet - or if, perhaps, it's not a passenger car motor and is a vette motor(!) then it's probably worth more to a purist. Would need more numbers off the block than just the casting number. Good find! Good luck!