Question for you: the car has a solid 4-speed (no overdrive) Transmission. All the synchros are intact and it shifts beautifully. What are the advantages or disadvantages of keeping the transmission, clutch and drivetrain set-up as they are already in place and functioning well?
There have been quite a few discussions of this issue in this forum - it's worthwhile to go through some of them to understand the varying viewpoints.
Assuming that you're keeping the rear axle basically as-is, there are three common choices:
- keep the transmission
- use a smaller fixed-ratio reduction gearbox instead of the stock transmission
- connect the motor to the rear axle with no gearbox (stock transmission or otherwise) between them
The advantage of keeping the stock transmission is mostly the choice of ratios - just as with the original engine, you can shift to keep the motor in a desirable speed range despite changing road speed. It also requires no changes from the transmission back (transmission mount, propeller shaft, shifter...).
The main disadvantage of keeping the stock transmission is that it forces the motor to occupy valuable space where the engine was, which could be used for battery. It's also unnecessarily heavy.