I am not sure that I can be of much help here.
I'd caution against a 3.9 diff in a car that isn't going to rev too hard. I've seen cars slow down considerably from going to a diff that is too short. Most Falcons would run better times on 3.45 or 3.7 diff ratios, rather than anything shorter. If you have the right cam and can rev to 7000+rpm then a 3.9 is ok. Otherwise you would find that the car would be reaching top speed well before the finish line and therefore hurting your ET.
We have just built a new combo for the dragster and had our first outing last Monday. We are now discussing a diff change. The reason being that we hit top speed by 1000' doing 125mph on 5800rpm and sit at 125mph for the rest of the track.
Just my 2c.
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