Converting to ralley dash using a non-ralley harness

By now you probably see the 2nd connector on the rally panel with 3 wires. So your standard panel has 10 wires total where rally panel has 11 wires total. That 1 wire difference is the only real hurdle. I dont have wiring diagrams for 71. I do have 72 but may not be the same. I could guess that the 2 switched hot wires, a blue with white tracer and a black, that the rally panel has is the difference but that is only a guess. If your 71 standard harness uses the black alone to feed brake warning lamp, oil warning lamp, and voltage limiter, you don't have to add the blue with white that feeds voltage limiter only in a rally panel. The black wire can feed brake warning and limiter. You'll likely end up with a solid state regulator outside the rally fuel gauge anyway. There is no oil warning lamp on the rally panel.
I know.... much of what I've stated doesn't jive with the drawings above. Those aren't my drawings.