I think you might be in the high 10's as it sits, since you have a 904 in it. I also use a 28" DR and a 4.10 gear, but with a 727. Take it to the track and find out, should be close.
Also, 4000-6200 RPM is a fat window....my converter flashes to ~5200 and I shift between 6500-6800 depending on the day. Make sure you have a good trans cooler and you'll be fine.
