the bellhousing on the w126 and w123 5 spds are pretty far apart. there are holes that just wont line up even if you move the starter wart which is what I was thinking. maybe a combination of moving the starter and an adapter plate, but I havent got my hands on a w123 5spd to check it. i dont know about the 4 spd case swap. it might be easier to tackle a chevy t5, except the rear has a slip yoke. I am also looking for other combos that might work.

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1982 300CD Turbo (Otis, "ups & downs") parts for sale
2003 TJ with Hemi (to go anywhere, quickly) sold
2001 Excursion Powerstroke (to go dependably)
1970 Mustang 428SCJ (to go fast)
1962 Corvette LS1 (to go in style)
2001 Schwinn Grape Krate 10spd (if all else fails)