65 barracuda engine swap

No, you should be fine with a stock motor. The 904 and 7.25 rear came from the factory behind a million 318s.
However, be aware of 2 issues:
The crank register of the 318 is larger than the '65 273- meaning you will need an adapter bushing in the crank in order to use the torque convertor from the '65. And you do need to use the '65 torque convertor, because the transmission input shaft changed size and spline count in '68(?).
If the 318 is '70 or later, the water pump inlet changed sides, meaning:
You will need a '70 or later style radiator
OR
You need to use the 273's timing chain cover and water pump on the 318.
Other things to be aware of:
If the 318 is out of a truck, the oil pan will not fit your Barracuda. Use the 273 pan/pickup/dipstick and tube from the 273.
Use all the throttle/kickdown linkages from the 273.
Member @Charrlie_S at one time supplied the needed adapter bushings- maybe he will chime in if he still does.