Hi, Mike. Your leak may not be because of the cap as much as the tank neck. Any radiator shop can desolder the stock tank neck and resolder a U.S-made neck for a few bucks if there's any doubt. This solves TWO problems: first, the factory neck soldering was typically rough- to the extent that the rubber sealing washer on the base of any rad cap could not reliably seal for long. The stock vented cap on the long tank is only a loose cover to keep any liquid inside from splashing out.
The Euro small tank pressure cap is rated 0.9-bar FIM (14.7psi x0.9=13.2 PSI U.S.) Running a 16 lb U.S cap is safe & will help raise the system boiling point a bit more. But do NOT take this idea to its logical conclusion and run a 22 psi NASCAR thermostat! Street radiator cores are only tested to about 15-16 psi and higher pressures may swell the transfer tubes and crack the soldered fins, thus ruining the whole radiator beyond repair.
Besides a 13-lb U.S thermostat actually releasing around 11 psi and reducing the pressure & boiling point in your system, your leakage appears to be coming out between the cap & tank neck. It runs down the tank (see the stains on yours), across the welded surge tank bracket, down the inner fender panel and right inside an oval frame hole. I've also seen the overflow hose spigot crack on the underside, using the same leakage path. The frame is normally sealed so cap/spigot leakage doesn't always hit the ground to alert you there's a leak. Instead, leaking coolant stays in the frame doing what water does best- it rusts the frame from the inside-out. So fixing that cap/tank problem is worth doing.
Couple of other points: my September '72-build L model Pantera had nickel-plated tanks inside and out, under a coat of flat black radiator paint. This was factory-done on L models to counteract tank rusting for owners that don't change antifreeze yearly. Earlier tanks were unplated; not sure on GT-5 & later.
It's also worth drilling our 3 recommended 3/8"-1/2" dia drain holes in the bottom of both rear frame rails, and in the extreme bottom of each leg of the horse-shoe shaped stampings that support the upper rear a-arms. Driving in the rain will throw water into that same frame hole; I've seen that frame rail full-to-the-brim after a rainy club run. Drilling drain holes prevents that from happening.
Dig around inside the oval hole with a long wire or something: I found broken spark plugs, rocks and old bolts in mine. Possibly the source of odd rattles you may hear at low speeds. The horse-shoe stampings are usually full of rust and dirt, ground by vibration to the consistency of tan baby powder. It holds moisture and rusts inside at the welds to the frame. Good luck.
