The AJ16 is the later (95-97) revised version of the AJ6. They make 10 more horsepower. Other than that, not really a significant difference between them (other than the ignition system) from the engine swapper's point of view.
As for swapping it in - not sure, I haven't seen any in person. However, people swapped the older XK series engine into those all the time, so where the XK fits, the AJ6/16 should.
Yea that's heavy stuff there. All I have on the Triumph's weight is 450lbs (not sure if it is a dressed weight or not). 550lbs looks like a bit much (the things flexed even with the stock engine).
All well, maybe a BOPR would work out better. Or maybe a SHO v6 ...
I really want to put a straight six in something. Having worked on and seen an AJ16 I can see how that would be a bad fit for a Triumph or MG (they are frigging huge).
Now I'm thinking of a Barra 6. How heavy / big are they? How hard would it be getting one over here?