Can anyone tell me the difference between the 240 and the 300 standard transmission (if any)? If there is a difference, what would it take to put a 300 4 speed to a 240 engine , and or a 240 4 speed to a 300 engine?

nothing , other than the pedal set, tranny, cross member, driveshaft sized to fit and a flywheel.
Having done two, I know that

no balancing is needed
no worries about holding up (have put 45,000 mi on one and 28kmi on the other)

ps- email me if you need help, pictures etc:-