Hi Pilot's daughter.
I think you may have to look at your dad's case on two levels: medical and legal.
I 'aint no lawyer, nor doctor, but maybe I can give a few pointers.
It's doubtful that you will find high grade evidence linking g-forces to your dads problems. For the research to be robust, large studies (or experiments) would have to be performed, and I'm pretty sure that wouldn't have happened.
Further down the hierarchy of evidence, would be some sort of "cohort" study, which may demonstrate that chaps who flew high g's, suffered more strokes. Again, probably difficult to make the link (the insurance industry may have more info).
Further down the scale would be demonstrating some sort of proxy outcome- some effect demonstrable in patients who had experienced excessive g's.
Bottom of the pile is "expert opinion" - in academic terms, it's the equivalent of tabloid journalism, but having said that, the courts do seem to give it some credence (in the UK at least.)
You can do some research yourself
PubMed home before spending money on a lawyer.
good luck, post your findings on here and we'll try and do some sort of appraisal.