Oil dipstick full mark

For the record, I've daily driven my Barracuda for 15 years, but I replaced the slant 6 that was in it with a 318 about two years ago and had a lot of problems with it. It's my first small block and I'm still learning. From what I gathered the 273, 318 and 340 oil pans from '64-'72 are essentially the same shape. Not sure about 360s. The early ones didn't have any stampings, the '67-'69 ones are stamped 394 and the '70-'72 are stamped 294 and those don't have the nipples. I'd guess that 90% of people, if they check their oil at all, depend on their factory dipstick. So there's got to be a reason why Chrysler marked their dipsticks where they did and that should be consistent at least with a 318 and a 294 pan.

When I filled 5 quarts with a new filter and marked my stick, it actually ended up being 1-1/2" above what @Dana67Dart measured. That is a significant difference and I'm trying to figure out why. At some point I suspected a coolant leak from the intake which could explain the higher fill level, but there was no milkshake. I was also wondering if there was gas leaking into the crankcase, but also didn't find evidence of that.
You've gathered incorrectly. This is why one dipstick and tube does not fit all. For example, try putting a 67-76 A body slant 6 pan on a 66 and back slant 6 A body. Ain't happenin, because the pans are shaped differently. There are subtle shape differences in V8 pans as well, so you have to be careful. These ain't Chevys where most everything is the same. We ain't that lucky. lol