That's typical DIY conversion practice, but when a serious manufacturer builds an EV - even on the platform of an existing gas-engined car - a conventional multi-speed transmission is not used. On the other hand, they're using PM AC (or in older designs, induction) motors with a high enough supply voltage that they have a very broad speed range is nearly constant power available.
Your calculation shows that top gear is too high if you only want to go 50 mph. A lower ratio would provide more torque to the wheels for a given road speed, and thus better performance, but a lower gear will also limit top speed... and so you see why a choice of ratios can be beneficial with a power band that looks like a pyramid.
That all makes sense, but it would be surprising if the AC-50 can be jammed anywhere close to the original gearbox position. Transmission tunnels can be modified, of course, but in the case of the Spitfire there are frame rails pretty close on each side. In a custom-built car, or one with a huge transmission tunnel (maybe it came with a big automatic transmission), there can be space to do this. A smaller-diameter motor can fit, too, but usually needs a reduction gear to operate at a suitable speed.
Also, think you'll want a little more than a shaft adapter. The "TransWarp" versions of Netgain's DC motors that are designed to be used this way have a housing (from the tail of a GM transmission).