This seller has been selling these kits for many years, and sometimes goes by the name "Gronk Performance." I did my own 2100 conversion for my J10 maybe 15 years ago, and I noticed these kits then. I found a junkyard carb, but there are a lot fewer of these carbs available now. Gronk used to sell rebuilt 2100s and 2150s, but I'm pretty sure he imports new 2150 repops now.
I would point out that the universe of CJ owners that have converted a 258 with a BBD to another carburetor is a lot larger than the corresponding universe for FSJs. CJs and Wranglers got only 258s (or 2.5Ls) after 1983 (?) and I'd say something like 95% of FSJs are V8s. You are probably already hearing from the only two members here with some experience in this area.
What do you hope to learn from members that have used this kit that has not been covered in other forum posts? I searched for Gronk Performance and the first hit was this one -
https://www.jeep-cj.com/forums/f2/gronk ... ade-21611/ - hard to see how this could be more positive. If you're unhappy with what you get, eBay offers a lot of protection for buyers. If you use your credit card, you can also contest the charges if ebay won't help you.
If you need a plug-and-play solution, the alternative is the Redline Holley-Weber kit; the moderator Stuka has this carburetor and recommends it. Available lots of places.
http://www.redlineweber.com/carb-kits/auto/jeep/ Available in either a 32/36 progressive version or the 38-38 (which may also be progressive - don't know).
if your carb shop was recommending a 2300 type Holley 2V, don't do that. BTDT. These carbs are terrible on the trail without a lot of modification; the aftermarket 2300 is a hot rod carb that will need to be tuned to your application. Plus, even when tuned they are terrible on mileage, in my experience. Great seat-of the-pants boost on the street though.