They were the same engine for all practical purposes. The the small block LA engine had a couple minor revisions (hydraulic roller cams in 1985, for example) a one major one (the "magnum" rework in 1992 that moved to stud mounted rockers, different intake manifold bolting, oiling through the pushrods, etc).
For all practical purposes, the only real difference between a 318 and and 360 of comparable vintage is that the 360s were commonly external balanced with either special dampers or weighted torque converters (finding the sketchiest way to accomplish something, don't you change, Mopar!). The 318 being a shorter stroke like the 340 (literally only a bore difference in basic geometry), it was easy to internally balance.
The Cummins engine wasn't "made to outlast any mopar" of the era. It was actually made for Case/New Holland. Certainly it would indeed outlast any of the small blocks, but it's not at mopar's request that this occurred. The only credit Chrysler can take for the Cummins engine durability was that they made engines and axles so weak that the engine had to be detuned all the way down to 160hp/400lb-ft to let the driveline of the first gen trucks hold up.
