adapter plate for 1980 300D auto to manual converion
 

W123 1980 300D non turbo.

First time Mercedes diesel owner, not a guru.

I am considering trying to source all the parts for a auto to manual swap someday.

I have the 4 speed auto with the red(unobtainium)modulator valve so I know one day I will have to do something...

So the parts I know that I will need are:

1: a gearbox, most likely 4 speed. Done
2: clutch plate/pressure plate
3: shifter assembly/shift linkage Done
4: clutch pedal assembly Done
5: modified driveshaft.
6: modified transmission mount
7: pilot bearing
8: manual motor flywheel Done

what else do I need in addition to all of this?

what is the differences between the one piece aluminum transmissions and the two piece aluminum/cast iron transmission?

also do you have to change the adapter plate between the block and transmission?
I have not really found a conclusive answer to this in the forums.

From a ratio standpoint, the iron box and the aluminum box are so close that there is no practical difference.
If the iron box is used, the existing intermediate plate ("adapter") will be used. If an aluminum box is used, the IP should be changed also, to provide the locating dowels for the aluminum box.
The second design IP is backwards compatible, the first design IP is not forward compatible.

so the 2-piece box will bolt right up to the existing plate and the 1 piece box will not because the plate does not have the centering pins. correct?

1980 is the year of change from iron box to aluminum box; early examples will have an iron box, and late production an aluminum box.
If the vacuum modulator of your auto box is on the left it may also have a second design IP. With that proviso, your statement is correct.

my modulator valve is on the passenger side towards the rear.

it is red, with a removable o-ring sealed cap and a adjustment beneath the cap.

so a first gen plate I'm guessing?

I've seen two styles of transmission, one with a slant back and one with a little square gearbox. Am I correct in believing that the iron box is the square one and the aluminum box is the slant back?

With the modulator on the right side the auto box is a 722.1xx, and most likely is attached to a first design plate.
The all-aluminum manual box is integral with the clutch housing (one piece).
Iron boxes were produced in two flavors: flat top (early), and slant top (late). Both have separate clutch housings, which are aluminum castings.
Therefore, there are three manual boxes: two iron/aluminum, and one all-aluminum.

slow in responding. Thanks for all the info.

I am well on my way to collecting all the pieces at this point.

looks like I am getting a one-piece off a 1983 240d with the shifter and linkages and pedal assembly.

pictures make it look like the alignment pins are on the bell housing and not the IP?
also wondering why the bolts for mounting the transmission won't align it when the transmission is bolted on?

also got a resurfaced flywheel so at this point I have all the hard to get/expensive parts.