You're confused, and it's easy to get confused on this because some S12 members throw around the label "C trans" too much.
So let me clarify one more time.
1984-85 and some EARLY 86 used the S12 "B" trans. That is the one pictured on the right in that last pic above.
Some (later) 86 and 87-88 cars used the S12 "C" trans. That is the one pictured on the left of the same pic.
The 240SX S13 and S14 cars used just that -- the S13/S14 trans. So as not to confuse anyone,
when you talk about a "C" trans, you are NOT talking about the trans in the 240SX. You are actually talking about the manual transmission found in some 1986 (usually late 86), and all 1987 and 88 S12's (Mark II... different bumpers, hood, and interior choices among other notable changes).SO... if you're going to run with a KA24E or KA24DE, and you have the S12 "B" trans, you have THREE choices:
1. Stick with the "B" trans... you can actually swap the KA bellhousing onto the S12 B trans with some work. It's been done by someone on here. YOU WILL NOT NEED TO CUT YOUR TRANS TUNNEL, the shifter will be in the SAME location.
2. Switch to the S12 "C" trans. Swap the bellhousing to the KA bellhousing (which I understand goes on easier. You will have to make a custom mount to the trans. The advantages of this transmission are that you will have a stronger trans, aftermarket shifter options, and YOU WILL NOT NEED TO CUT YOUR TRANS TUNNEL, the shifter will be in the SAME location.
3. Switch to the S13/S14 trans. Make the "washer-style" trans mount as shown in my build thread in the "Club-S12 Garage" section. Cut your trans tunnel to accommodate the new shifter position. The advantages are your shifter is CLOSER to you now, which some find more comfortable (which is why it was done for the S13/S14), and you have a stronger trans, and you get aftermarket shifter options.
NOW, if you have the later Mark II S12 with the S12 "C" trans, you have TWO choices:
1. Keep the S12 "C" trans and just swap the housing. All the benefits as mentioned in above point #2.
2. Switch to the S13/S14 trans, with all the results mentioned in point #3 above.
IF you have a 1984-85 or earlier 86 S12 with an AUTOMATIC transmission, then you will need a custom mount for either the S12 B trans, S12 C trans, or the S13/S14 trans. There is no exception. All three require a custom mount when dealing with Mark I automatic S12's.
IF you have a later 86 or 88/87 Mark II automatic S12, you can make a very SIMPLE mount involving a straight piece of steel, and run a Mark II S12 C trans. Or you can make a custom mount and run the S13/S14 trans.
THOSE are your options.
IggyEGuana has a great picture for his custom trans mount, he went from Mark I automatic to S13/S14 trans.
read this thread, it has PLENTY of GREAT info by Waylon (IggyEGuana) as well as he has included links to yet more useful threads and pics:
http://club-s12.org/v3/index.php?topic=18215