Assuming it's the ECM, you could be out for <$1000 sourcing a new ECM and such from Ken. I wound up buying one from him because the wheel/tire combo I wound up selecting (well after Mallett's demise) was a smaller OD than the stock setup, and I wanted to ensure my odometer and speedometer would read correctly. Mallett did lock the ECMs, but being across the country from their shop, I would have replaced the ECM so I could work on it locally anyhow.
If you decide you want more power, or the baseline provided by Ken doesn't solve the issue, you can be fixed up for less than $1500 all-in.
Ken's ECM didn't work out of the box for me, but that's mainly because there are a few possibilities for how some things may have been wired up depending on who did the V8 conversion. That was all solvable via email, and Ken provided updated calibrations that I loaded up from my laptop, meaning I got it all working in my garage.