i have a 73 charger w/a 400 cid motor. i had the motor and tranny rebuilt for performance and now i'm pushing over 400 hp and a tranny to handle that. problem is i have the original differential gearing and would like to know if anyone reads this has any ideas about changing the gears and the ratios/types i need to handle the power train.

thanks

 

Should have an 8.75" in there with a "....489" case (casting number on the case) behind a 400 CI with most likely a .323 or .355 gear. If it hasn't been molested a tag will tell the ratio.
Easy to tell the 8.75", just look under the rear of the car. If you don't see a differential cover bolted on you should be good to go for the HP.
If by chance there is a bolted on cover on the back of the differential, you'll probably be looking at an 8.25" which will have issues with the HP.
Check the Tech sections in ...........
forbbodiesonly.com
or even ........
forabodiesonly.com