Pretty sure logistics have been mentioned. As to the other changes, there were field modifications for improving frontal armor and flotation. The former was merely welding on an additional plate to the front that IIRC some crews did on their own initiative, the other was an Army modification that involved putting "duck bill" connectors on the tracks that widened them. The 76mm I think was decided on because of logistics, that gun was already being used by tank destroyers like the M10. Not sure how well a 77mm would fit inside a Sherman turret. Granted, I'm pretty sure it was smaller (hence the reason for it existing) than the 17pdr which went into older Sherman turrets, but that was also an ergonomic nightmare and Americans have high standards for ergonomics.