An M46 w/o OD (if it's factory, all bets are off for DIY jobbies) is an M45.

Gearshift supports are the same, the shifter hole is identical for all transmission variants so at least it'll fit properly.

Other considerations:
You will obviously need to change the driveshaft and support crossmember, and I'm not sure if these early 240 have the crossmember mounting holes in the right place for the M47, so you may need to drill these. M47 may have a different number of splines on the input shaft so you may need to change the clutch plate. Unless the clutch is very new, have a good look at it and the throwout bearing, and replace the tiny pilot bearing as a matter of course. They're cheap and a nuisance when they go (noisy, rough shifts). Otherwise, simple bolt on conversion.

Bram