There's actually room for 5 golf cart size 6v batts under the hood. Which is good, since there's room for 7 in the hatch back, meaning you preserve your rear seat & keep the f/r weight distribution relatively balanced.
20 miles and 35 mph @ 72v will be a piece of cake! In the summer, I was able to squeeze about 30 km (~18 mi.) from the ForkenSwift's tired, used 48v pack (that's best case, not avg.), and can hit 60 km/h (~40 mph) with enough road.
FYI:
http://www.evalbum.com/564 - 72v Metro convertible for comparison (though he's using 12 x 12v batts in 6 buddy pairs).
I'd expect you could hit 55-60 mph fresh off the charger with enough road.