With the TKO I would go no higher (numerically) than 3.50 or 3.70. I switched my 3.89s out for 3.70 also due to the TKO. With that kind of power and the short 1st gear of the TKO, this is what I would do if I were you. One reason I wished I woulda done the T56 in hindsight, but oh well.

And since it's a street car I would do the .64 overdrive - same as I did and for the same reason.

Anthony's setup of T56 and 3.89 is an ideal setup to me.

I'm pretty happy with my 3.70's, but I'm running a T56, so it's a bit different than the TKO600.

Since your BBC is going to make in the neighborhood of 600ft-lbs of torque, I'd lean towards gears in the 3.70 to 3.90 range with the size tires that you think you're going to run. I think that would be a great street setup.

Just something to think about
 

You need to figure what RPM you will be turning in your top gear at 65 mph. I can tell you from expirence that depending on your motor it could suck.

Although mine is completely different than yours, this is what I ran into.
I have 19s on the back with 285/30/19 I believe it is 27 tall. 3.80 gear and a T56. At 65 in 6th I was turning 1500 RPM. My car idles at 950. If anyone slowed me off of the 1500, my car would lug and I would have to shift into 5th. Big pain in the ass. Changed the gear to 4.11 and now in 6th at 65 mph I am turning 1750. Does not sound like that big of a deal but it is huge cruising on the interstate in traffic or trying to obey the law. I can slow to 55mph with out a down shift.

He's looking at a fairly radical camshaft so he will want to keep his overdrive in the 2000rpm range minimum with a big block chevy. Especially since it will be carbed. Mine is comfortable at 2k at around 60 mph. That makes it comfortable for most highway and all freeway jaunts. I agree with flash on the low 1st gear. I wouldn't want lower gears than a 3.73. I considered 3.55 gears but ultimately decided on 3.73 due to wanting to be in overdrive around 60 mph. The T56 is a whole different ballgame.