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The way I'm going to do it on my '73 Dart Sport is to first get the transmission bolted up and, from under the car, push the shifter up against the floor and mark the center of where the shifter hits the floor.
I'll locate the hump based on that.



That's how I did it on my '77 Aspen using a '73 A-body hump. I have some pics and a brief write-up on the process HERE , which will be very similar for an A-body.

I wouldn't be overly concerned about getting the hump perfectly postioned, as there was surely some variety from the factory from car to car. Once you set the hump in place and start opening up the hole, it will darn near locate itself.

Remember, you can always cut the opening larger...