Nathan,
I am not speaking officially for either Texas Thunder Speedway, or Heart of Texas Speedway. I am talking as an "insider" perhaps, but not officially for either track.
If'n I was you, right now, I wouldn't build an I-Stock. Yes, they are having car count issues at both the tracks you mentioned, and at other Texas tracks as well.
In fact, I asked Brett Root, Vice President of Operations for I. M. C. A. about this on the first Dirt Track Insider. He said that I-Stock numbers are falling, particularly in Texas. Listen to the show for the reasons he thinks that is. I, for what it is worth, agree with him.
http://kmacsports.com/DTI.shtml Click on the link, and select "The Premiere episode of DTI" and click that link to listen to the show. Brett was my first guest, and gave a great interview. And you get answers straight from the guy running I. M. C. A.
So, if you don't build an I-Stock, then what? I personally would go with an I. M. C. A. Southern SportMod. A lot of the "missing" I-Stock drivers have shown up in this class, as well as a lot of other great racers. Again, listening to the above link will tell you why.
It is a shame, as the best race of the night often times is the I-Stock feature. Heck, their heats can be killer as well. I love the I-Stocks. I don't want to see them die out. But if you listen to Brett's interview, you can see where it is maybe time to let them go.
Nothing against SportMods, they put on some great racing as well, but I-Stocks are my favorite class.
Again, speaking as an insider, a Dirt Track Insider, if you will. But that is how I see it.
BTW, you can call Pat Wilson, Promoter of Texas Thunder Speedway (and I-Stock driver at Heart of Texas Speedway) at 254-290-6262
Bill "Sarge" Masom
I-Stock fan