Need some help. 1971 K-20

[lookn4awhitetail]

Hey Folks,
Im new here, and trying to get some info on a truck I recently bought. It has supposedly an original 350 in it. And it looks that way, as it cosmetically leaves a lot to be desired. But runs really good. Shifts nicely too. Ive already changed the oil and filter and just am trying to get some help identifying the other majors. Im thinking this is the SM465 in there, and then Im assuming this is the GM 10 bolt rear end. But the low line fill port on the diff cap kinda threw me off. Just trying to see some opinions on what everyones take is on it. I want to check all my differential and transfer case fluids, but want to make sure I have the right stuff on hand before cracking open the check ports.

Also notice in the one, where the speedometer cable is going into the transfer case, It looks like there is a good bit of seepage going on, and I am hoping that is where a lot of the minimal loss is coming from. Is this a common leak, do you have to replace the entire cable. Haven’t owned one of these style trucks since I was a kid, and then I surely didn’t know what the hell I was looking at. It was a 2WD as well so a lot of other things to make sure and keep tabs on. Just looking for some guidance,