Check out the
EV Album for other Swift/Metro/Firefly conversions
Anything north of 7 inches should do the job. For 50 MPH you'll want at least 96 volts nominal (resting) pack voltage. I've done 62 MPH (100kph) in a 3 phase AC Wheego Whip at 96 volts. It scooted to 50 mph just fine, but the last 12 mph were on the weak side; to sustain 60, one more battery would be advised.
Carlos, what is your budget? Are we talking salvage special or state-of-the-art? How long do you plan on "sustaining" 50 mph? Range is inversely proportional to speed. Also, on lead acid batteries, you will loose 1% of your range for every degree the battery pack is over or under 77ºF. If you're in a desert, cooling is advised. if on tundra, heating is needed.
Electric cars are fantastic, but you really need to know what you're getting into.
~Jim
EV mechanic,
Minneapolis, MN