I agree that you need to drop the lower oil pan and look at the pickup tube.
Perhaps there is a remote possibility that you kinked or bent a fuel line, reducing fuel flow. It doesn't seem likely but feeding the engine with diesel from a soda bottle under the hood and seeing if performance increased would quickly rule it out.

I believe he said he removed the exhaust and checked it for problems.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13