I have a 5 point harness, I would not install a 4 point without the sub belt even in a Pantera as you can slide out under a 4 point in a heavy impact. The harness does restrict your movement and accessibility, I am 6ft and the seat is far enough back I cannot turn the ignition key with shoulder belts on. I am thinking of going back to a more traditional 3 point harness for accessibility and I'm past the 'racer' look.
As I am the only driver once the harness was set for me I have no problem, it does take a little for various passengers to get everything straight. Although not installed yet I have a flat firewall kit that would make access to the engine bay a non-issue even with the should bar in place. You can modify your own bulkhead for easy access relatively easily and I am sure there are discussions on how to do this if you search the forum. In the later cars with the belt reel on the lower bulkhead it is just one more screw to remove the belt cover before removing the bulkhead, a non issue IMO.