Well after a lengthy test drive, it turns out that the bearings were not the final solution to the issue of periodic vibration on the passenger front of the Targa. Knowing that this had something to do with rotating mass and sensing that it was being transmitted through the brakes, the only remaining component was the Caliper itself. The caliper could be binding due to the piston becoming stuck once it got hot. With 40-year-old calipers, I decided to take the route of replacing the caliper rather than a rebuild kit. I could then rebuild the caliper sometime later.
The parts arrived within a week, and I completed the simple job of replacing the caliper and bleeding the brakes within an hour. I choose to put brand-new pads in, so now the front brakes are mismatched, but they performed well in the short test drive. I then took a much longer test ride down to the Radnor Hunt Concours which is covered in the main blog. All was well on the round-trip and I think I can finally declare the issue resolved.