[bret a ewing]

See if this makes sense.........

The TKO 600 with a 3.55 rear ratio is very close (within 2 mph across the gear and rpm spectrum ) to a wide ratio toploader and also utilizing a 3.55 rear ratio. This was the generally the standard ratios back in the '60s.

So, depending on your cam,

You can either run: 52mph @ 2000rpm with a .82 5th gear
or..
You can run: 67mph @ 2000rpm with a .65 5th gear

Hope this helps.

Regards, Bret.