Was a special housing used for the sure-grip assembles? My car came with 2.76 open gears. The housing is noticably different then the replacement center section I have.(I'm going to use a 3.23 sure-grip...optional in 73)

Here is a picture of what's supposed to be an Oct 72 built center section.(3.55 gear) https://board.moparts.org/ubbthreads/upload2/6435961-SureGripMarkings001.jpg

Here's my original Oct 72 built center section. You can see that there is a cast pad in the center towards the top. The area under the pinion snubber has less material then the other housing. Mine has a small 9 cast into it that is facing right side up. The other housing has a large 9 on its side. There are other differences in the ribbing and the casting numbers toward the pinion.

I've got another picture of a 73 Challenger with 3.55 gears and the housing is the same as the first picture.

Chrysler has more than one plant that made castings, just as they have many assembly and engine plants.

Darren here is a shot of the unit that came out of my car it has the 9 but no pad up area like the one in your picture. It is an Oct 72 car as well..

With some (all?) later 489 cases, you have to grind some material out of the case to make higher numerically gears fit.

To answer your question , no there was not a special casting for sure grip rears .