Thanks to everyone for the replies, I gained some insight. As is often the case, overall value seems to be determined by a number of additional factors such as location, etc...just like a D0VE block, or a core engine, and so on.
Based on the feedback (range value) in this thread, I came up with a detailed proposal that provides him with an immediate, partial payment that keeps his head above the water for the time being, and then something more down the road based on the selling price of the truck. This way, we both take a small risk and friendships don't get weird over an infinitesimal amount of cash.
Thanks very much to all of you for your feedback and insight,
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Performance Components for the 429/460 Engine Family