A good start would be to check the archives here at Brickboard... lots of previous discussion on the subject...

I'm not sure if one needs to replace the whole pedal box, or just replace the brake, and add a clutch pedal. Of course, you'd need the clutch hydraulics (and be sure you have the proper bell housing for the same). You'd need a flywheel. The transmission tunnel on a auto 122 is bigger, and has no hole for the shifter, so there'd at least be some cutting to do. Perhaps the biggest issue though is the crossmember, which needs to go in a different place than the automatic one, and there are no threaded holes where it goes, so there probably would be some welding/cutting involved. Oh... and your driveshaft will probably be too short unless you have just the right M41 with overdrive... some of them have the same length driveshaft.

Definitely more of a project than doing the same in a 140, which is pretty much a bolt up operation.

That said, it can be done. And there are folks here who have... but I ain't one of 'em.

-Matt
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-Matt '70 145s, '65 1800s, '66 122s wagon, others inc. '53 XK120 FHC