496 gallons is close enough to 500 that I would just divide everything by 100.
Black Malt .5
Carafa III .4
If you can't get any info on efficiency, start with that and see how close it is, tweak it from there. If you can
find some efficiency or alcohol content, then I would only change the 2-Row to hit the gravity for your system.
That's what I would do anyway.