Alright here's some pics of the door. You can see the header is solid all the way to the wall plate. Before I thought there was a seam at around 4", but that actually turned out to be just a crack in the lumber. It's a 2x8 header (or a solid post, couldn't tell through the minimal gap between the top jamb and header). This is also a 2x6 framed wall. So after viewing this setup, it looks like the builder decided to use a full 2x8 width from the top plate and cut the jambs and door a little short instead of cutting the header. Probably wasn't a big deal back in '72 when they built the jambs from scratch anyway.
I would need to shave off around 1 1/2" from the bottom of the header to make the new door fit. Shouldn't be an issue, right?