I am assuming you don't have a service manual for your year and model of Challenger .
They are worth every penny and are available from many automotive liturature dealers and you can even buy reprints as the originals are quite expensive . E-bay is another place they show up from time to time .
You will find all the exploded veiw illustrations you need to locate all the hidden studs , nuts , screws ...etc . Unfortunately as I don't have an E-body at this time , I lent my manual out to a friend . I'm sure someone out there can scan and post the illustration for the dash from their manual .
The best tool I've found for removing the hidden nuts from under the dash is a really cheap ratchet with a flexible cable / shaft that is stiff enough to make it go where you want it yet manouver around obstacles . I got it for Christmas a few years ago ( you know , one of those , draw a name from a hat deals where you don't spend more than $10.00 ) It has a ball shaped handle that ratchets and the shaft is about 12 inches long .
Just Dashes is the way to go , it's not cheap but the dash is guaranteed for life and they look awesome . If you have a 1970 Challenger you know about the "one year only" stitching they had . Well Just Dashes has duplicated it perfectly .
Your other option is to install one of those dash caps . It will do the job , but it's just a temporary fix that doesn't look right when you compare it to the rest of a nice interior . But it all comes down to personal choice and finances .
As Chryco said , make sure you remove your dash VIN before you send your core to Just Dashes . Put it in a safe place and then reinstall it on the restored dash pad .
From time to time you can find the original style pop rivets for sale but I've always just drilled out the original rivets from the back side of the dash pad . Now just drill the center of the rivet enough so you can squeeze the flared portion of the rivet tight enough to pull it up through the dash pad and VIN plate . When I reinstall the original rivets , I force them back through the VIN plate and dash pad , then by carefully backing the exposed ( top of dash ) portion of the rivet head with a peice of wood , I flare out the backside of the rivet on the under side of the pad . For extra strength I put a blob of 5 minute epoxy on also .
It's all under the dash where you can't see it so neatness isn't as important as if it was exposed .
I hope that helped , Good Luck and if your serious about working on your vehicles get a proper Chrysler / Dodge or Plymouth SERVICE MANUAL .
Have a Great MOPAR Day , Doug