I replaced my 4.08 with a 3.69 rear axle (complete swap - the cost of the cogs and the time to change them out exceeds the cost of a good used complete rear axle) and it resulted in 'longer legs'. However I suspect the effect on the mileage would be quite small. And in your case, changing to a 3.69 would not be worth the trouble. There is the possibility of going lower still, but you would need a sedan rear-end and if you have an automatic, that would for sure make the car pretty sluggish on take-off. I would stick to what you have.
Cees Klumper in Amsterdam
'69 white 280 SL automatic