I have bought a 91 Z28 , that originally had a 305, it now has a 350 with the original t/5 behind it and i have been told that it isn't strong enough to take the 350.
My question is what can be done to the t/5 to beef it up to be ok behind the 350?

 

Just be careful when shifting and don't deliberately try to break it. The later, World Class, T-5 is a bit stronger, but GM still didn't feel it was idiot-proof enough to go behind a 350. However, lots of people use these with 350s and don't have problems.