I've had a 76 Ramcharger and a 78 W150. Both trucks had factory 400 engines. They both had the dipstick tube run to the pan with the 90 degree fitting. Actually, every factory rear sump big block pan that I've seen, has that fitting. I'm not saying that that's always true, just what I've seen with 70's trucks engines I've worked with.
Matter of fact, way back when I was dealing with this issue, the truck pan didn't have a provision for the block mounted dipstick. I was swapping a 440 in place of a 360 using a wrecked 400 equipped truck for parts.
