Triumph Repair: rear end ratios vs gear ratios, overdrive gearbox, gear ratios

Question
QUESTION: Howard,
I have a 1979 Spit 1500 stock.  The car does NOT have overdrive.  I believe the rear end is a 3.89  and the gear ratios are 3.75, 2.16, 1.39, and 1.  I enjoy the "out of the hole" feel of the car however, I am pushing some rpms when getting towards 45-50mph.  Should I change the rear end to a 3.27 or a 3.63; or change the transmission to an overdrive...or change the 1:1 4th gear to something like a .8:1  I would like to keep the peppiness of the car however, lower the rpms when traveling in 4th gear.  Just wondering what your take on it would be.  Also, do you believe changing the rear end to either an anti slip or a locking rear differrential has any benefit?...(for street use)

ANSWER: Hi Billy,
An overdrive gearbox makes more sense because then you have both (if you can find one).
Howard

---------- FOLLOW-UP ----------

QUESTION: Would I have to buy the entire transmission, or can I install an overdrive on my existing transmission.  Also, any benefit from the locking diff or anti-slip?

Answer
An overdrive can not be installed on a std trans unless you get a trans mainshaft for an overdrive installed in the transmission. You would have a better chance of finding a complete transmission with an overdrive.
The only place a "anti-slip" diff is useful is in snow, sand, mud or racing.
Howard