Putting a 3.9 Magnum in my 1973 Scamp

As already mentioned, your /6 trans is a 904 but won't work anyway as the bolt pattern is different. Because the 3.9 was based on the small block, there is a lot of commonality.

Easiest thing might be to just rebuild the 3.9 trans. Especially if it is a 4 speed OD. Done.

Seems everyone automatically wants to increase the power. You could send the cam to Oregon Cam and get it reground. You could get the FI to work or if it's carbureted, go bigger.

I love the 225 in my 64 Valiant but if I ever did a swap, I'd go Pentastar 3.6. 300 HP stock.