Has anyone ever tried to replace a bad 2003 4.6L engine with a good 2002 4.0L?
Does it bolt up and fit properly?

 

it is the same physical size the only difference is in the stoke, if you were so inclined you could put the reciprocating assembly from your motor into the 4.0 and have a 4.6. but there are plenty of trucks that started life with 4.6 that now have 4.0 in them, there (sadly to say) weren't enough good 4.6 to keep up with the demand.

 

You must make sure you get a SAI (secondary air injection) 4.0 from a DII though, if you do not your check engine light will be on all the time.

 

or swap your heads onto that short block

 

Direct plug and play, as for the ECU, you can have a mechanic with the proper scanner reset the ECU to control a 4.0. You should remember that if you ever sell your Disco, you must disclose that it now has a 4.0 so the new buyer won't come back at a later date and demand his money back.