I probably should of expanded on my answer...
Your car should have a 8 3/4" rear end and the outer housing, axles etc. will be the same as the 383 rear end. But! The center section may be different.
As
@1970FuryConv has said, the yoke size needs to match. You'll have two choices... The yoke will either measure 2 1/8" across or 2 5/8" across. They do make adapter u-joints that go from one to another, but I'm not a fan of doing it that way.
The other issue is the size of the pinion shaft. If your 318 rear and the 383 rear have the same yoke size, everything will bolt up and you'll be fine... But! There's a good chance you are going to have the more desirable larger pinion shaft diameter with the 383 rear and the smaller pinion with the 318. Not a big deal, but if the new rear is the larger pinion, then it's a not a bad idea to swap center sections.
MyMopar - Mopar Forums & Information - Chrysler 8-3/4" Rear Axle Guide for more info on the differences.
You can identify the center sections by the number cast in the side. In the example below, this is a small pinion center section. 741 is the 1 3/8" pinion, 742 is the 1 3/4" pinion and 489 is the 1 7/8" pinion. Unless they've been swapped (it happens), your car will have a 741 or 742. Most likely, the 383 will be 742.
View attachment 319220
This isn't saying anything about gear ratios... Again, you need to look at what you have and what the 383 rear is. If it hasn't been messed with (again, it happens), there should be a tag on each rear where the center section bolts in that tells you the gear ratio.
Since the guy is selling as a package deal, you may be money ahead to buy everything and sort it out as you go.