If a 74 is the same as a 69 then you don't need a different driveshaft. The
overdrive replaces the tail housing in the non-overdrive transmission, so
the overall length is the same. Also there is no vacume switch.
There were two different types of overdrives used in the MGB, the D type
which is commonly referred to as the early or old type used up to ~67, and the
LH type used from ~68 on.
I agree with Will Zehring, a complete transmission is worth a fraction of the
cost of it's constituant parts. As an example MOSS wants ~$300 for the sliding
member for an LH O/D. You can buy a complete O/D new, yes new, from MGBHive
in England for ~195 pounds, @ $1.56 = 1 pound thats ~$300 for the whole
The switch that works the O/D on 69s is built into the turn signal stalk so
all cars have them, even have the wiring from the switch to a connector
down by the starter, (ever wonder what that yellow wire thats hanging there
was supposed to be doing?).
It may not be as hard as it looks to swap in a good O/D transmission, finding
a good one is the hardest part.
69 MGC-GT green \ I must be crazy! Somebody please buy one of these so
69 MGC-GT red / I'll have time to finish the other one.