I think with selling something that's low volume in a niche market, your reputation is all you have. It seems like your carb setup almost makes them as specialty a piece as the top casting itself. So leaving a complex carb setup (something that is a dying art in and of itself) to a buyer spending money under the pretense of "ease of use" sounds like a lot of a risk.
I'd at the very least include a primer in the instructions giving a brief explination of why you need to make the exact modifications to the specific carb in regards to design and volume. Include the same text on the site next to the product that only comes with carb and top casting. With how fast word travels in this internet world, you should think about covering all your bases.
I think the idea is great though! Beautiful execution and an innovative way to cover a market that needs a product.
I can't count how many tunnel ram setups I see that are for eye candy alone.