Dave:
Here is a gear ratio calculator that does a pretty good job
http://www.368s.com/tools_rpm.phtml, just plug in the numbers and go. Even calculates tire diameter for you.
I agree with Ernie about some people not looking at the whole picture when they should be.
In my case I'm holding out as long as possible for the new TKO600 with the .82 or .83 (depending on who you talk to) 5th and a 3.07 rear. The TKO600 has a higher 1st gear than the TKO, TKOII and Richmond and comes close to the toploader wide ratio numbers with the addition of an overdrive 5th.
If you go with the standard TKO or TKOII you can bring the rear end up to 2.88 since the first gear is so low but as Ernie mentioned your 5th at .68 is going to turn some really low rpm's and may well be pretty useless. The TKO .82/.83 5th would probably work out to be a bit more streetable.
Just my $0.02.
DonC