IIRC a 8 3/8 is the same as a 8 1/4 just called 2 different names. I call it a 10 bolt. 9 1/4 a 12 bolt. next up is the 12 bolt cover on a D60 very similar to a 9 1/4 cover at first glance.

I would have a look at the ring gear for yr/ratio to see if it has been replaced. a lot could have been worked on since 79.

I have replaced a lot of rear axles as a whole going with what I could find. a 10 bolt is always easy and cheaper to find but now days they are getting hard to locate. a 9 1/4 is hard to find that in good condition or it has/need a rebuild they tend to hold up if the crush sleeve is replaced with a solid spacer.

72-74 RC-4x4 truck would have a 8 3/4 as well. I have one from a 72 w100 with 489-3.91sg (cone) FYI-same hosing/axles as a C-body car.

I have relocated a couple 9 1/4 axles from 73-74 newports to trucks with a perch move same width flange to flange as truck chassis.

so when looking for a mopar rear axle I look at ALL of them.

just installed a 67 polara 8 3/4 in my stepside to tuck 10 rims and have the A-body 8 3/4 in the 88-440 truck tucking 12" rims.