Truing rod big ends shouldn't change the length of the connecting rod. It is done by grinding the same amount off both the rod and the rod cap contact surfaces. Then, a special hone is used to bring the hole back to true. Since equal is removed from each side, the total length should be unaffected.
The manual recommends coating the FO8's with WellSeal, which is a very thick shellac type sealant. It was developed by Rolls during WW2 to hold the engine together with 40 inches of boost. When I heard that, I decided it's good enough for me and my Triumph!