518 46rh help please

How is reverse?
Reverse and Drive both use the hi-drum. In reverse, the pressure is unregulated and should be ~240 to 300psi @1600rpm;IIRC

But I agree your governor is not working correctly. The question is; is it the governor or is it the circuit, or is it the 2-3 shift valve or did you forget the #3 check ball .That ball separates the Hi from the reverse. If you forget that ball, hi-drum pressure gets dumped straight into the pan,thru what would normally be the reverse circuit. So check your reverse pressure.
Reverse pressure if intact ( 2 balls; #3 and #2; and a properly detented manual valve,), equals line-pressure, equals pump pressure; usually in the range of 230 to 260 but can be as high as 300@1600rpm. However, line pressure also feeds the TC, thru the TC control valve........ so if the TC is hemorrhaging pressure..................... ; anyway, just check the reverse pressure. Since you have the OD unit off, make sure the governor is properly assembled and that you have the right one for your rear gear and tire-size, and that the flyweight is properly sprung and not sticking..
BTW, you do have your TV pressure cable hooked up and adjusted, right?, aka the Kickdown mech. I'm pretty sure the TF-II still requires it

Happy Hunting