Classic/Antique Car Repair: Transmissions & Differentials, rear end gears, torque converter

Question
How about changing the torque converter out to an oem spec?
-------------------------------------------
The text above is a follow-up to ...

-----Question-----
I have a '68 GTO with a 400cid and a Turbo 400 with a 2000 stall torque converter, hate to say it but am unaware of the gears on the diff. no marking on the housing.  I have electronic ignition with a 600 cfm Elelbrock.  I have now power coming off of the line without power braking, just no power.  Since I am coming off of the line at a dead stop do I need to change the torque converter out or change rear end gears?  I can cruise down the road easily at 70+ mph and am afraid if I change the gearing that I would not be able to do this without turning high rpm's. The car has never performed the way I think it should for being what it is.  The car was set up this way when it was purchased, engine and trans only.  Any ideas as this is my first hot rod and am learning as I go.  thanks, kevin
-----Answer-----
Kevin;
HI   well you could change the gears , but yes you will rev on the hi wway.  you might want to look into a cam kit. ... and headers.

Answer
Kevin;
HI well as far as the rear end there is a way how to tell what gears are in it with out taking it apart, but sorry I forget. some how you mark the wheel than rotate it , etc. but I do not think tha ToQ.conv. will  not change much, if you come off the line with no RPmS up you will go no place the RPM s must be up a bit for you to get up and go, I would put a cam in it. a cam will do wonders.