95 Mustang 5.0 Fuel/tuning Issue

It's possible to make your injectors behave like the next larger size up by increasing the fuel pressure. It's likely that the MAF was calibrated to 42lb. injectors. 30lb. injectors being run at 70psi (instead of 39.5) will make them act like 40lb injectors - though probably a little lean but within the limits that the computer can adjust for. This can be done without tuning. Sounds like they may have been looking to push everything they could through that nitrous plate, which is understandable. Always get the bigger injectors when in question.

https://www.rceng.com/technical.aspx

Now why the C&L didn't respond properly to your bringing it back down to 39.5 with sampling tube adjustment I don't know, other than the fact that C&L's aren't all that reliable. You sure they're 30's? You used the purple sample tube (think that's right for 30's on a 76mm). FYI flow ratings for aftermarket injectors are typically at 43.5 and not at 39.5 so adjust to 43.5psi fuel pressure as your baseline (with the vacuum line disconnected).

The injectors might have also been run so long at 70 that they no longer respond well at low pulsewidths at a lower pressure (poor idle).

How to address this depends on your goals for the car. If you want a reliable daily driver and aren't going to be putting all kinds of fog through it, then ditch the C&L, get some 24's and a matched MAF or Cobra computer (or the factory 19's and stock MAF). Or go back to what it was running before for the short-term.

Checking if it was 'tuned' is pretty easy - passenger kick-panel and check if there's a piggy-back chip on the computer's J3 service port. That's the only way to tune these guys, there's no flashing the ROM or anything like that.