Putting a 3.9 Magnum in my 1973 Scamp

I know most everyone will tell me why not do a V-8? Valid question, one reason is I only bought this Scamp because it is different! I'm a GM man normally, but this Scamp just caught my eye, so I bought it. I have been debating (mostly with myself) on what swap I want to do. Right now it has a 225 slant six and it's tired and ready for a rest! I found a decent buy on a 3.9 Magnum, close to my home that was running and driving when the guy shelled his trans. Any thoughts would help me out here, I have never done this kind of thing before and want to be able in the end to say "I did this!" I have an automatic trans, but don't know how to tell if it's a 904 or 727? Thanks for letting me join your group and GO MOPAR!! Lol....by the way, my wife and I just purchased a 2020 V-6 Charger! Love it.