One final thing to try that should tell you if your current setup is providing sufficient travel to completely disengage the clutch, is to put the car on a lift. Raise the car. Have your mechanic bleed the system and put in fresh fluid. You will be sitting in the driver's seat. Start the engine. Have your mechanic walk under the car and watch the travel of the clutch fork while you depress the clutch and put the car into reverse. If it grinds (after first engaging a forward gear) have him push the clutch fork a tiny bit more with a lever or piece of wood and see if the shifting improves. If it does, you have found your problem. You need more travel. If you currently have a 1" master cylinder and a 7/8" slave cylinder, you should have about 1 1/4 " of fork travel. That is more than enough travel to ensure total release of the clutch. Most of us shift perfectly with about 1" of travel. If it still grinds in reverse, then you have an internal clutch problem.
You're going to have to pull the trans and figure out why your clutch isn't completely disengaging. All it takes is a single millimeter of warpage or misalignment to cause the problems you are experiencing. Watch this youtube video and you'll see the disc only moves a total of about 2 millimeteres from completely engaged to completely disengaged
How a clutch works - internals of transmission and clutch assembly - YouTube. That can happen for a variety of reasons. Hot spot on the flywheel or pressure plate, slight imperfection on the disc, dragging pilot bushing or throwout bearing. The only way to completely eliminate the problem is to yank the trans and replace everything.
All the fixes you are doing have all been tried by many others on this forum, myself included. Hundreds of SPF owners have perfectly shifting TKO 600 trannys with engines that generate the same or more heat than yours. They all have the same 7/8" master and slave cylinders, same hydraulic line placement, same header locations, same brake fluid, yet theirs shift perfectly every time. If you've eliminated the external factors, and it sounds like you have, it's time to go internal.