The bond cars (Jag & Aston) were front engined, rear wheel drive cars originally. As I remember from "the making of..." documentary, the smaller windsor engines were fitted primarily to create space for the rocket launchers etc.
Now as far as why would you do All-Wheel-Drive (as opposed to 4WD) on a GT40... You would do it to improve traction above and beyond that of the 2WD drive cars. How would you do it? I'd hate to think. I suspect that the only "off the shelf" components that you could get would come from the AWD Diablo (if you could get them to sell it to you), then start throwing money at it. Now the theory is that it would be even quicker around a race-track, but of-course there is the weight trade-off. Is it worth it? It was all the talk in the early 90s that it would be the next big thing on all mid-engined super-cars, but it went nowhere. So I suppose the answer is NO.