replace 65 valiant 273

Yes you can use the 318, mounts will bolt right up. Your converter will work, but you MAY have to make a bushing to go in the register of the crank shaft, I don't remember what year the diameters of the converter hubs changed, they may be the same or the 318 will be larger, hence the need for a bushing. Not a big deal, done all the time.