[ChibiBlackSheep]

I haven't tuned a 3900, but you would turn off a part of the tune that allows the car to limit the speed/rpm cutoff by cutting the fueling to the car. Either the RPM limiter or the speed limiter.

And yeah, you let it upshift, wait for it to hit the converter and blip a little throttle to get it out of that and when you are at like 3k past the converter hit you should be fine to hit it on the dyno.

You could also be super safe and adjust the tune just for the dyno, make the downshift speeds something low, like 25, so it won't downshift