I planned on putting in a five-speed when I bought mine, but they don’t need it; four speeds are enough. The only question is what gearbox is in it now? It’s an early car so you should have the terrible Moss gearbox with no synchro on first but.....you say it’s got a 4.2 motor, so maybe they put in the matching all-synchro box with it? If it’s still the Moss then I’d say change it out - originality doesn’t matter because of all the other modifications on the car. If it’s the later box, drive it for a bit and you’ll probably find (like I did) that the four speed is just fine.