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Did you look on the back side near the firewall?
If your truck has a oil filter,look where the oil line goes into the side of the can about 2/3rds the way up. It should be on a "tee" with a single wire attached. Some of the earlier ones had the sender and tee down where the oil line comes out of the block.( behind the l/s head )If it isn't in these places, someone might have eliminated it for some reason.
Mine is screwed into the block, behind the driver's side head, where the oil line from the filter screws in also (there's two holes, and the line and sending unit are side by side). If your wiring harness is correct and original, you should be able to follow a white wire coming out of the harness that goes between the starter selonoid and voltage regulator, about where the left side head is that white wire should drop down there to behind the head.
Also, the original sending unit looks different than the replacement/modern kind. The old one is maybe 1 1/2 or 2 inches in diameter, sort of mushroom shaped on top.