AMC was founded in1954, their first v8's were designed and built by Packard. The rest of AMC's V8's were designed in house also the I6's were also AMC designs. The 258 was used by IH as you previously stated the 360V8 and the 258 I6 went with Jeep to Chrysler and were used by them in the Jeep line. It is entirely possible that IH built these engines for AMC and probably did, as you stated AMC farmed out most of their components. Having said all of that they are all AMC designed and very good engines. Long story longer if you have one it is worth keeping and using.
Jerry I love reading your posts you are a wealth of knowledge.
