actually the difference is in the torque converter,318 would be internally or neutral balance, ie no weights on the torque convertor, a 360 is externally balanced, hence they use a weighted torque convertor.the flex plate on oem stockers is determined by the size of the convertor, there are 2 V8 flex plates, small and large outside bolt pattern.you can use your passenger side motor mount bracket on the 360 as it is the same from the 318, however 360s have a narrow spread on the drivers side mounting lugs, plus 1 is in a different location than on the 318, you can bolt the drivers side 318 bravket to the 360 block and fab up a piece to match the 360 bolt location or buy the68-72 340 a-body specific engine mounting bracket,mopar dealers used to be able to get them a few years ago...so did year 1.