Putting 340 in car do i need a new transmission?

with doing this yes you need a trans for that motor. any way the rear end on your 225 wont be able to handle the tork so you will need a bigger rear axel and the radiator is too small so you will need a bigger one as well.and chances are you will have to measure the length of the drive shaft space after the motor and trans is in to find out how long of a shaft is needed theres alot of work with it so good luck