OOpsie daisy! I reread your post to see that you have the later 4.0 engine. These 4.0/4.6 are pretty much a straight swap. The difference between the 4.0 and the 4.6 is the stroked crank.
There are other things you can do to these, like installing a high lift cam, putting in Ford or Chevy pistons with different compression ratios, porting the heads, installing headers...aluminumv8.com has some interesting ideas.
Want 400-plus horsepower? Talk to those guys.
1966 109 5 door wagon 300Tdi "spermaceti fueled"
1994 RRC LeWiB "ruining the air behind me"
1968 2A 88
All my troubles are Rover