The 304, 345 and 392 are all IH and different from anything similar in displacement that AMC offered. IH used the same engines in their smaller truck line. The 392 was never put in Scouts but like you said is often sourced from larger vehicles.
They did use the AMC 232 and and AMC 258 six cylinder engines in addition to the Nissan diesels, but that was it. If it wasn't a six cylinder, it was a IH engine.
They make some hop up parts for the IH V-8's, but not a whole lot. They have a lot of stroke and are way overbuilt for more diesel like low end torque and durablity than racing. A four barrel intake, a cam or two and maybe headers would be about it, they by no means have a SBC following.
Later Scouts had D44's on both ends, as opposed to D27/D30/D44 mixes prior to 1974. D60's were never an option.
My dad has a 1967 1100 pickup with a 304 that my great grandfather bought new, they made good scary simple trucks back then. Ours still has a glass sediment bowl fuel filter, a throwback the company's tractor line no doubt; as is the inner door handle, unavailable from anywhere unless you ask for a 1066 tractor cab handle... which is identical.