You can use the same motor mounts as the 273, BUT-
318s (both LA and Magnum) have a larger crankshaft counterbore than the 273- which means you'd need to have a spacer bushing made (or buy- but I'm not aware of any current sources) in order to properly fit the '65 torque convertor. Or alternately, have a custom TC built.
There may also be issues with exhaust manifold fitment on later ('73 and up?) 318s and Magnums, the driver's side manifold interferes with part of the head. But that's a whole different thread.