Your cable jacket install looks good with nice big sweeping turns. It may well be your speedo; the simplest way to check is to borrow a known-good instrument from another owner, hook it up and drive a ways. It need not be put in the dash for this. Pantera and Mangusta speedos of the same ratio are interchangeable and have green ink-lettering on the case back to denote the ratio. There are two ratios and two different cable connector threads, so 4 possibilities.
The right-angle ZF speedo adapter is tricky to install: the dual-thread nut must be screwed all the way up the adapter body, then installed by screwing it down until things bottom lightly on the ZF drive. If the adapter is not installed this way, the system can drive off the extreme tips of the fork-adapter in the assembly and will often break one side of the fork, over time. Over-tightening the nut does nothing except make one tired. Sealing to prevent tranny lube leaks is done by an o-ring in the assembly, not the tightness of the nut.
45-yr-old NOS right-angle adapters normally contain brick-dust that once was lubricant and are not made to be service-able. Note also that if an expensive right-angle adapter is actually broken, they're often repairable. I've fixed more than a dozen over the years with various ailments.