Do you have a BW35 or BW65?.
The 35 can be split from the bell house which makes it easier to remove ( and less awkward )
65 needs to come out as one.
Both can be removed from under the car just watch the fan on the radiator when you remove the box.
If your shifts are late and sloppy then the cable is too slack, if the shifts are harsh and early then the cable is too tight.
Just ensure the engine idle speed is set 600-650 rpm when warm ( lower if possible ) and your fluid level is correct.
Also worth checking the linkage as this has a...
Just check the front of the Sun shaft, when it hit the floor it might have dislodged the relief ball valve in the middle of the shaft itself ( worth checking its free moving anyway ) I doubt it came out but good to check it.
Plate colours and prefix code ( before serial number ) denote what that particular transmission is allocated too vehicle wise, it also denotes as you have mentioned different engine applications.
Each prefix code is allocated to a certain vehicle manufacturer and each prefix code / plate colour...
If that's your car pictured in the avatar ( Nada spec ) then my guess is you have a 9FU BW35 fitted, they do have a check valve spring in the valve body but the spring is very weak and just not up to the job especially if it has compressed over time as it's a really flimsy one.
Early BW35,s had...
Way to clean for it to be my thumb Harvey :)
Looking at the face of the sun shaft it is showing signs of wear, looks like the needle bearings are shot too.
Unusual for the front needle to wear badly it's usually the rear one that suffers most.
Be a good time to convert to dipstick and filler...
"There is an electronic version of the 4HP. I wonder "how electronic" is actually is. I had it on my BMW e34 5 series. It had a "sport" and "normal" selector but didn't appear to drive much differently to any other auto apart from the shift points."
That's the early type Peter...
Not used the composite gasket, always stuck with the cork which work well when fitted correctly.
Make sure the sump pan is flat along the mating surface, if it is not then make it so.
Use hylomar ( universal blue ) on the pan face only then refit and tighten the bolts until just over hand tight...
I don't think the issue is with the valve body swap, I would try and set the throttle valve cable correctly, or check that it is.
Also ensure your engine idle speed is set between 600-650 RPM before you start and check all the linkage relating to throttle set up including the spindle bush are as...