OldIHC

Looks pretty solid, not too far from Me

https://www.facebook.com/marketplace/it ... 1936599513

The cab is surprisingly intact, but didn't see pics of inside. Has a curious full flow oil filter adapter mounted on a GRD engine. I would think it would rob too much oil flow from the engine unless there's a restriction somewhere in that piping.

Hard to say without seeing how it is plumbed. There are spin on bypass filters as well. Without seeing where and how it is connected it is hard to know what it is.
Could have inserted a filter in between the pump and main oil rifle or it could be spin on bypass plumbed like the original or it could be a botch job like you think.

sitting right where fuel pump would be,

still gonna be a bypass system unless they did something inside to divert oil directly to filter unit

simply connecting a spin canister to factory line inputs only makes changing oil easier,