Only just spotted your reply, sorry to be slow off the mark.
Agreed the PRV isn't the problem. I saw a mix of throttle rods from two suppliers when I came to look at your car but didn't realise that throttle linkage and rods came from the same supplier. In any case, I struggle to understand how a supplier can sell rods that are too long and need to be cut down and have lock nuts added by the end user. THey should fit the car straight out of the box.
The Moss ball jointed rods I fitted to my car about 10 years ago were the correct length, and fitted straight in ready for balancing the throttle plates. My only slight criticism is they would have been better with a finer thread, making small adjustments more precise.
Hope this makes sense, and best of luck,
1970 Triumph TR6, CP-series
1972 Triumph GT6 Mk3
1980 Triumph TR7, Sprint engine
1970 Triumph TR6R 650cc
1962 Triumph T100SS 500cc