Here's the story John.
The 1970 FSM is not a complete manual. A person had to obtain a supplement in order to have the full story. The manual I purchased directly from the dealer in 78 included the supplement at the back. Russ has that same manual. Apparently those looking at Scott's site are not reviewing the supplement.
1970 was the first yr of the Isolated Field Alternator. The info for it is not in the main manual, but it is in the supplement, as noted by Russ. There are 12 pages of service procedures and tests for the charging system. After reviewing the diagrams there, it is obvious at the time it was written the wire colors were intended to be different than how they actually ended up being installed in the truck. Unfortunate, but really no big deal as it is only a couple wires, under the hood. Your mysterious orange wire is one of them.
Now that we have cleared that up, just hook it up and forget it...B)