I haven't converted a car to overdrive, but I have put a non-overdrive engine in an overdrive car and noticed a few differences.
The clutch is a completely different diameter(in my case the car was late model), you will need a flywheel and clutch to match the overdrive transmission. You are probably on your own for wiring, you may have to make your own small harness/wires. The drive shaft is a different length, and the trans mount is different.
Sounds like a lot, but it all comes together quite easily if you have the parts. Plus, if you are really anxious to drive, you can do it in stages, like refurbish the trans, and install. Then hook up the wiring when you have time.
Adam H.
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1973 Triumph Spitfire