Overdrive trans or change rearend/gears

[69chevy lwb]

what about the 3speed overdrive manual transmission it would bolt up to his factory column shift and with a little wiring and a overdrive cable you would have 3 speed with an overdive that automatically shifts in and out for passing gear the pre 67 ones have a non syncro first gear with a 4 bolt side cover and the 67 and later have a 7 bolt side cover with syncro first and also share the same transmission case as the saginaw 4 speed so you can change the main shaft and the overdrive unit and have a 4 speed overdrive heres mine its a 61-64 impala trans non syncro first i built a floor shifter for it but decided to go with a stock 3 on the tree instead since no one has them anymore