In a W126, it's a day job. I've done many of them. In a W140, I would imagine the harness and vacuum routing is different, but I would think a few days max. I've actually started wondering if I should part with a couple W126's and pool some money with some cash set aside and spring for a 06 or 07 W211 E320 CDI. Each time I drive to Atlanta, it's 1,000 miles roundtrip, and those cars seem made for those long routine drives with the MPG they get.

__________________
1987 300SDL #1, 1987 300SDL #2, 1980 240D, 1982 300SD, 1994 S350, 1990 350SDL, 1991 350SD, 1985 300D, 2005 E320CDI

Gone, but not forgotten: 1981 300SD, 1982 300SD, 1987 300SDL, 1983 300TD, 1980 300CD, 1981 300SD #2, 1987 300D, 1987 300TDT, 1980 300D