I gotta 70 camaro 4spd and it needs a throw out bearing,.now can just leave the bellhousing on the engine and take off the tranny from the bellhousing and reach inside and replace the bearing, or does the
bellhousing need to come off  

yes.. you can just drop the tranny and reach in to change the throw out bearing... be sure to prop up the motor so you dont break off the dizzy when the tranny drops... sometimes they hit the firewall and snap.....

you can also get a plastic plug for output shaft so all the gear oil does not drain out when you tilt it .....be sure to refill it when you are done  

If the throwout bearing is worn out, I'd recommend pulling the bellhousing off anyway and inspect the pressure plate and disc to see if they need replacement.