I have just installed a CBC TH350 in my 400/69 Firebird.
Is it necessary to connect the kick down cable to the carb or can i get away with manually shifting to overtake.

Harmless to trans to run without, and downshifting manually will work fine if you don't mind doing it that way.

It's also cheap & easy to just install it and have it work properly...universal kits like this come with brackets and everything included:
http://www.summitracing.com/parts/lok-kd-2350u/overview/

You do need to drop the pan to install the cable, so the time to do it was during installation...but now it'll just cost you a few more quarts of tranny fluid in addition to a kit like that above.

Thanks for the link...the cable is on the trans already so i guess i'll just get the brackets

Then absolutely there is no reason to not hook it up. $25 or less and 15 minutes of putzing around, getting a bracket fitted and the cable set/adjusted right, and it'll kick down like it's supposed to when you stomp the gas. You can buy universal/adjustable brackets pretty much anywhere. Just buy one by looks more than anything. Got mine from local O-Reilly's dirt cheap...pretty sure it was a Mr Gasket brand I saw sitting on the shelf.

Tons of options popped up with a quick Google search:
http://www.google.com/#q=th350+kickdown+bracket&source=univ&tbm=shop&tbo=u&sa=X&ei=FOnWUculOoakrQGitIHgCg&ved=0CCoQsxg&bav=on.2,or.r_qf.&bvm=bv.48705608,d.aWM&fp=739421dc22eac3be&biw=1344&bih=743

Look for some universal brackets, I found one that is adjustable for the kickdown and it mounts to the back bolt of the carb.

http://www.spectreperformance.com/index.php/throttle-cable-bracket-w-tool.html

Or you can get a OER brkt and drill it out for the part that holds the cable.
I tried a lot of these brkts and they are all for Chevies and dodges.

Cheers...thanks for the info guys