I would be seeing where a domestic 3.8L V6 could play.
In your personal situation, we have an IT catch-all: any car legal for IT is legal to run STU. So, your Mustang is good.

On the other hand, if you want to build a 3.8L Mustang or Camaro specifically to the more-liberal STU rules, we've classified those cars into STU at 3200# (see "Alternate Vehicle Allowances" table at end of STU specs.) Look at that table and you'll see that the precedent has been set for approval of 3L+ requests on a case-by-base basis, primarily based on expected engine output.

[That w]as what had me concerned I'd be chasing a moving target if I just read the GCR and didn't read all the threads on the forums.
We don't have a hard-and-fast output upper limit for STU, right now it's one of those "you know it when you see it" kinda things. But, since the competition weights are based on engine output, the ratios will remain consistent, and outliers will most assuredly be adjusted as needed. In fact, we are using data acquisition on STO at Nationals, and STU is in the pipeline for the same once things start to shake out.

Short of a major shake-up in those P/W ratios (no change currently being considered), I don't see any major changes in the general ruleset.

GA