it is if your 4.6 sounds like an early model duramax.

 

4.0 and 4.6 blocks are identical. Except the 4.0 blocks were machined better. I'm moving all my rotating parts and heads from the 4.6 to the 4.0 block. So I will still have a 4.6.

 

Jafir is right. You need a map sensor in the top of your fuel pump, you need to run 3 wires from the computer to it. Then you need to install a vent valve under hood where the big pipe runs from the dmtl pump. Then you need to wire the vent valve. I have done it and it is easy. For the plugs in the head, the hole in them is a 9mm Allen. You could jb weld a cut off Allen key piece in each of the 4 inserts. You can swap the ecu and bcm out of any 99-02 disco and wouldn't have to program it, but the odometer will blink on and off unless you swap the cluster also.

 

not all 4.0 have better blocks, better tooling.
look for color coding
that "all 4.0s are better blocks" seems to be more of a perpetuated rumor. it may be statistically common, but no evidence supports this. just hear say.