Given the white smoke (BIG clue), I'd consider borrowing another diesel truck to jump yours with, and doing a compression check. You have a leak between the cooling system and the combustion chambers. Could be cracked heads, leaking head gaskets, cracked block, or...?
Also wondering if perhaps the oil you were seeing in your radiator was diesel fuel rather than motor oil.
Compression test will tell you which cylinder(s), then try retorqueing the head gaskets and test again. If it hasn't improved anything, then pull the heads and inspect the gaskets at the low cyl's. If the gasket looks okay, check the heads.
I'm with Aces & JAN... I think your gaskets may have failed. I'd rule that out before going on to try to diagnose a more severe problem.
You can find an exploded diagram of the motor at Peninsular Diesel's website. Look for their "manual", it's a pdf download, iirc. If there's something more specific you need, I can post pages from the FSM for you.
Suggestion: Take pics and post them here as you go... you'll likely get better troubleshooting help.