It's not hard to find a black or white sided battery. I usually go with all black and then put the topper on. Napa has all black batteries but I usually end up at Costco for an Interstate battery, they are all black now too, and for about $75 and a 6 or 7 year battery, you can't beat it. Throw a topper on it and it fools most people.
If you're looking to gut an original for a smaller battery, it's doable. PSMC's did it all the time to save weight over the nose. Looks stock but inside is a smaller battery wired to the original terminals. The rules have cracked down on that in the last couple years so I don't know if you find many doing it now. I almost did it on mine when I was competing and racing my firebird. We got to the point of taking a bathroom scale with you to the battery store and start weighing batteries, you get funny looks from the counter people
You would be surprised at the differences in weight between brands of batteries that are the same series. But this is another subject entirely.