Two things to keep in mind as you decide which way to go; the Gear Vendors OD ratio is not nearly as good as the 518. .69 vs .78. That is a big deal. And the 518 is available with converter lock-up.

There are calculators that will compute the RPM difference with your particular inputs.

Now, the converter lock-up may not be happy with the full power of the big block, but that really shouldn't be an issue. How often would you want to go to wide open in OD?

The 518 probably is somewhat more complicated to find and assemble for a big block, and nearly as expensive to boot. But the better final drive ratio makes it worth the extra effort in my humble opinion. Check out the calculators and see if would be worth it to you, too.