The Jeep 4. 0L is a worked AMC 258 CI AMC engine that has been around since the 60's or early 70's, maybe longer. They also made a 232 CI version of it also. AMC also made a 307 (?), 360, and 401 (?) V8, and I was told once that all their engines shared the same bell housing, making engine conversions pretty easy.
I have owned a 1973 AMC Gemlin with the 232, a 1974 Gremlin with the 258, a 1976 Cherokee with a 258, a 1978 Jeep J10 PU with a 360, and a 1987 Jeep Cherokee with the 4. 0L.