Aerodynamics and body design evolution from 1980s Le Mans Prototypes / IMSA GTPs to today

I'm curious about the evolution in the shapes of the Le Mans Prototypes / IMSA GTPs, from 1980s to today.  The 1980s cars - Porsche 962 for example, have a smooth, fluid look to them, while the modern crop of cars are much different - a bubble cockpit and 4 wheel pods, a much more complex shape with slots and channels everywhere.  You can't judge the aerodynamic performance by the looks, of course.  

My questions for you aero experts:

1) was the evolution in shape driven by improving aerodynamic performance, rules, or (probably) both? 

2) What are the advantages and disadvantages of the shape of say the 962 compared to the Toyota TS050?

3) How did changes in the rules drive the shapes we see today?  I know the vertical fin, also seen on F1 cars, was rules driven, how about the bubble cockpit  and 4 wheel pods?

Subscribed! I can dig out all the diecasts and slot cars for body profile shots of these generations if it helps.

you could dig through here:
http://www.mulsannescorner.com/

or you could buy the book by that site's owner:
https://www.abebooks.com/9780760330692/IMSAs-Legendary-GTP-Race-Cars-0760330697/plp

As for part of Question 3...
The Vertical fin is there because of the rules mandate.  However, it's important to remember that their inclusion in the rules was because of a couple of crashes mid-corner where the car lost some control and then lifted off.  These results encouraged the ACO to perform an aerodynamic investigation of the dominant prototype shapes through a greater 'beta' (i.e. side wind angles) range, including (iirc) direct crosswind and backwards.  The 'big honking holes' over the wheels were added after further investigation of the same potential for side-slip instability/divergence.

A lot of the changes were on the undersides.

The old flat bottoms gave way to tunnels, diffusers.   Big changes on the underside of the cars ahead of the front wheels.  The underside of the noses became diffusers.  And don't rule out the advent of super strong splitters out front.

Thanks for the links, I have some reading to do!  It seems like the Allard J2X was the leader in the evolution to the shapes we see today.

These days aero in top-tier racing is as much about optimizing air flowing through a vehicle as over/under it.  CFD modeling, CAD and structural analysis has really advanced car design; composite materials, CNC milling and 3D printing have made it possible to deliver parts that would have been impossible to build in previous decades.

I know that rules play a huge part in what can and can't be utilized and ultimately what shapes the cars. I.e. ground effects in the GTP cars were no longer allowed and cars had to be open top which led to the WSC era with the Riley and Scotts, Ferrari 333SP, the Chevy Cannibal, etc. 

I know in the early 2000s Audi had some ingenious solutions to the rules. The rear wing end plates were not allow to connect to the rear quarter panel for LMP900s. Audi developed a rear wing with end plates that stretched all the way down to the quarter panel with a gap so small you could barely get a sheet of paper through it. 

http://www.benzing.it/enrico.profili.htm