Steve, our '72-1/2 L model uses 4-1/2 degrees of caster via offset bushings and shaved upper ball joints- a bit less than your car at 5-1/4 degrees. We use no power steering and when she was driving, my 5'3", 115 lb wife said our heavier steering was tolerable at low speeds and not noticed above 40 mph, even with 245-50 x 15" front tires. High speed handling was much improved, as you found. So it sounds like you and your Pantera really don't need P/S.
To recap, I did a few things to make steering a little easier for her without going to the trouble and expense of power steering or re-engineering the suspension. First, I installed 8" Campy rear wheels in front. These are built with 1" less scrub radius than 7" Campys- the pivot point of the 8" wheel/tire is exactly under the wheel/tire centerline. The 7" Campys are offset outboard by 1".
Second, I installed a MOMO Protitipo steering wheel that is a little larger in diameter than a stock Pantera steering wheel, thus giving her more turning leverage. The larger wheel does not interfere with either of our legs under the wheel, although some owners do find interference with larger OD wheels.
Third, I use minimum settings for camber and toe-in with stiffer polyurethane a-arm bushings, and have Hall sphere-ball fittings on both ends of both anti-sway bars, to take out any possible binding. I also had the stock early seats rebuilt for good support and modified them for adjustable 'rake'.
These easy tweeks seem to allow tolerable manual steering effort on our car even with max-diameter tires. We both formerly autocrossed this car in N.CA without arm-pain; she won her year's class 4 times and I won my class 3 times against 'Vettes and Camaros with P/S, back in the '80s/90s.