If the engine replacing the 318 is an external balanced engine, then a B&M flex plate is all that is needed.
The extra power from the larger mill can kill the trans due to years of service never seeing more than 250 hp. The normal 904 beind later year 318's is up to task for "MILDLY" built engines. Putting a strong built engine behnd a standard 318 trans is not the greatest idea. The trans should be "Worked up." for this.