The only items requiring replacing is the ECU chip a piggyback two chips on 96 OB2 vs a single chip for 95 and older OB1 engines to properly fuel 4.0 or 4.6 engine as well installing a 4.6 airflow meter. You will have slightly less maximum power and but still run normal if you use your original 4.0 airflow meter vs installing the larger 4.6 airflow meter.
Stock factory chip is already way too lean vs an aftermarket chip like a Mark Adams chip.
http://www.tornadosystems.com/
This chip properly fuels as well improves all around engine performance, it's not a maximum dyno hp only chip but one that functions and fuels overall from at idle to maximum rpm's. A practical daily driving chip also returning better mpg depending how you drive. Lead foot no!
I replaced the 3.9 engine with a 4.6 short block in the 95 D1 with a chip, cam and true double roller cam timing set a upgrade as well retaining the distributor allowing full control of the ignition advance of when and how much in degrees advance. While your at it, port match the intake to heads as they are sickly out of alignment costing you another quick easy (your call) performance improvement upgrade.
This 95 (OB1) also the last year with distributor vs electronic advance and coil packs of 96 and newer OB2 engines.
I can run with the the original 5AM airflow meter which fueled the 3.9 engine or swap it out with a larger 4.6 airflow meter,
http://www.roverlandparts.com/2003-
2004discoveryiimassairflowsensor46l.aspx
I have no connection or gains from the above parts suppliers, just a DIY wanting more power and fun driving experience D1 with upgrades.
BTW, if I were in your shoes a 4.6 would be an instant swap vs going back to a 4.0 engine.
An extra 50 ft/lbs torque and app 40+ horsepower comes in handy hauling around an already under powered 5,000 pound Disco. Swap added smiles to the miles for me. .....~o&o>......
