It will reduce about 10% through the range, unoticeable on the road but very useful round a circuit . Most 3.9 conversions are the cwp out of a granada which is ok , the motorsport cwp's with the larger teeth tend to last a bit longer. A large hp car will lunch this equally as quickly tho !
If you intend to do top speed runs this will inhibit "very" high speed but you'll get to peak revs a bit quicker .
Cheers
ps it may be useful to know what type of Diff unit it has as i may have a use for this !