If you didn't swap over the MB radiator to go with the diesel engine, I'd say you need to probably take a hard look at that, you need to make sure what ever radiator you have is up to the task. Easiest way is to use the MB radiator, just a matter of fitting that into the truck.

Replace the thermostat and the gasket, they cost about $20 or so. Be sure to check the old and new thermostats in a pan of water on the stove to make sure they open at correct temperature and reach full stroke by correct temperature.

Did you change the injection pump timing at all? Getting the timing off can lead to the white smoke and may also be related to the over heating.

What does the white smoke smell like? Sweet smell = coolant, unburnt diesel smell = injection problem (timing or bad injector(s)).

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC