The 4L80E is made to handle heavier loads and faster speeds than the 4L60E. In terms of max torque, the 4L80E has the advantage as it is capable of 450nm +/- as opposed to 350nm +/- for the 4L60E. Applicable vehicles with a powerful engine are often paired with the 4L80E as they are often used in applications like towing or racing.
Stand alone computers for the 6L90e are available around $900. Also going to need a different computer for your 454 that's calibrated for a manual trans because the programming between the 2 different model years won't work together. 6L90e computer will need a few inputs from the engine, TPS and RPM. F.