Hello I just got a 83 monte carlo with a sbc 400 and a TH400 turbo trans I'm wondering on what I should do to make it fast either new engines/tran or work with what I've got

 

I know absolutely nothing about SBCs, but just in general for questions like this, you'd get far more useful responses if you specified your budget and goal (either hp, 1/4 mile trap, etc - whatever the primary use will be).

Without that info you could get stuff from 'add some headers' to 'blown alcohol bbc swap' and everything in between.

 

He is right!
But if I was gonna do it I would put in a nice crate engine, manual transmission and make sure I had a decent rear end assembly. 4 Link the the rear end.
New adjustable shocks all around and you will probably have to rebuild the front end suspension.