lol I hope that's not what you think I'm doing here, I'm a mechanical engineer myself and actually took a course on internal combustion engine theory and design. I'm just trying to get inside the minds of the engineers back then and understand the reasons behind it. However it looks like Jim Lusk and 67 Power Wagon gave the best answers so far, I'll take it.
Good article Hemaniac I think I read it before but it gives really good info on the later emissions heads and debunks the myths about them being undesirable (they actually respond better to porting than earlier heads like the 906 and 915). Didn't see any specific mention of the valve location though maybe I missed it...?