As was stated earlier on the stock converter you're not going to get much better than what you've done. My car picked up half a second in the quarter mile after my Circle D converter went in. Oh and with 3.15's.
I had many Mustangs over the course of 30 years and I'm on my second Challenger with the 392. The Challenger is more roomy and better looking in my opinion over the current Mustang. The 392 puts out a lot of torque but won't be as fast. But it's a better overall cruiser in my opinion.