I should clarify - if Chryco thinks the bolt pattern was different on a '70 than on '71-'74, it may well be - he is the guru around here.
I don't know for sure - I have a '70 with a manual trans.
If you win it, hold onto it until you can match it to your car. If it works, keep it. If it doesn't, e-bay it to re-coup some of your money and then find the right one. If you already have a crossmember and they both fit, keep the better one and offer the other one here or put it back up on e-bay. I rather suspect that if the patterns are different, trying to change it will not be worth the time. After all - it's a clone, not the real thing and no matter how much you change it, it won't change the value of the car and you'll never know the difference between the two crossmembers when you're driving it.
If I can offer a little friendly observation/suggestion - you've mentioned several times that you're new to Mopars (welcome!) and have a lot to learn. My suggestion is to give yourself time to learn. Why not restore what you have - a '74 - to allow you to learn about the cars and then work on making the changes to a '70 clone. Of course, you've already changed the tail panel...