[Dimis]

This question can become a case of paralysis by analysis.

Its been mentioned already that engine specs and rear end ratio will help with the decision, as would intended use. Like if you are paning to track the car for fun or for competition or at all.

That all said, they did race these things with only 4 gears 1.0 ratio in 4th, so any 5th gear is an improvement of sorts. That's good for up to 150mph+/- depending on rev limits and ratios and which way the wind is blowing.
That's pretty much going to entertain most people.

Not knowing the full details I'd suggest the .82 ONLY if you are racing for competition.
Else if your not then a 0.64 wouldnt hinder your track enjoyment, and would most likely benefit your hwy cruising speeds with a lower, quieter rpm and a fuel saving of sorts.

Fwiw: I totally ignored this advice and went the 0.82. when it was time to bite the bullet (and cos Brent said go the .82) but I'd probably be equally happy with the 0.64.