My
is that you want to see what sort of look you want. I have the Rallye gauges in my car and there have been times when I wish I had the exact set up you have (standard gauge panel with aftermarket gauges) for a racier look. The factory look is unique and nice and all but it looks a lot more plain.
As far as the tach inacuracy, if you go with the Rallye gauges, are you still going to keep the aftermarket tach? Sorta like wearing sneakers with a suit. I know lots of cars are set up like this but if you are going with the factory look, then I would pass on the aftermarket tach. Just my opinion...
As far as the correct style, when was your car built? Early cars sometime got the 70-71 Rallye dash gauges. My 1972 Challenger Rallye SBD was 8/17/1971 and I have the 70-71 style gauges from the factory so depending on your SBD, the 70-71 gauges COULD have been in your car.
I assume you car came with the standard gauges to begin with so originality to the car may not be too big a deal. If you don't want people going to your car and saying, "Hey, you got the wrong year Rallye gauges in your car.", then go for the 72-74 style gauges. Might be cheaper than the 70-71 gauges too.