A lot of the body position of the older GP bikes and World SBK's, had to do with the bike itself. The tank design, the narrow-'ness' of the seat/tail, ergos of the clip-ons, etc.... There's a reason current motogp bikes' tanks are flat. The rider is able to stretch his body/chest/arms across the tank, instead of around the tank which allows them to hang off more.
It was also due to simply a lack of knowledge about the physics involved when corner at high speeds. We have learned a lot over the past 20-30 years.