Rob - I did the diagonal 5 point like you mentioned. 1 5/8" DSE pre-fab, sanctioning body non-compliant, not tied into any frame rails but fabbed strong enoungh for "what if". I also worked w Cris to prep the car for sub down bars, trangulated/reinforced under the dash. This was done for tortional ridgidity.
Brandon - My opinon only, but a good seat combined w good belts and a good halo is a good start. HANS is a whole 'nother level up, welcomed on track.
If I ever build a track only car, it'll have a sanctioning body compliant cage and I'll strongly consider a HANS system.
Cage systems in a PT car is a very personal choice and something is better than nothing...
