When that engine was designed and built the engineers had 10W40 or 10W30 in mind (10W40 preferred).
Unless you have had a hotter cam and or stiffer valve springs installed I would not be concerned with the lower level of ZDDP specified in many of today's passenger car oils.
My first preference would be 10W40, but if I wanted to experiment I would be inclined to try a high quality 5W40 or 0W40 full synthetic to take advantage of the development of oils that provide better lubrication during start-up and warm-up.
I enjoyed Panterias very much. I was a Lincoln/Mercury Tune-Up Tech in a dealership that sold a lot of them and I worked on them often. Changing plugs and points was quite an exercise. One bank of plugs came out best from below while the other bank best come out from above. I'm tall, 6'2", and still needed to stand on tip toes to access the distributor to replace and then adjust the dual points. Checking or setting the timing required removing the bulkhead behind the driver for access to the timing marks.
I invited the customer to go on a road test with me after finishing a tune-up on his Pantera and he accepted. During the road test a Corvette pulled up next to us. The customer spoke up and said "go ahead". I resisted the temptation.
They really are fun cars to drive.