We checked the clutch several times, and it is fully disengaging (this was one of the first tests TREMEC had me preform the first time).
Out of curiosity, and in an attempt to assist, please expound on the procedure executed?
Also, perhaps your typical technique of engaging 5th leading to the difficulty?
Realize that attempting to discover the solution on a forum from afar is not really likely, but perhaps, some food for thought may be conjured. And , you asked.
Sorry for backing up in the post, and my intentions are not to badger, but, generally in order to arrive at an accurate conclusion one must be systematic in ruling out distractions. For example: the proper fluid to be utilized would be a Dextron II ATF product (not "gear lube"), and no one particular brand should be at fault in this issue. Also, the instance of the grinding during shifting, this being supposedly executed with the clutch disengaged, therefore engine power potential is somewhat irrelevant, thou the boxes can be for racing applications somewhat R.P.M. limited. I believe one should find the Tremec "Z" code T5 transmission a fine product, and capable behind most street intended Mustang/Shelby vehicles with S.B.F.s, with the potential of say 11 second quarter mile performances.
But, it ain't working for you, why? Well, the problem seems to be that upon the engagement effort, the synchronizer-blocker-clutch ring as part of the synchronizer assembly fixed to the counter shaft, is not clutching the 5th gear and thereby not "synchronizing" the rotational speeds of the components. Well, no s##t Einstein, but why?
It could be just defective relationship of manufacture in the applicable components, as implied previously, this is often the most difficult to establish "in the field", but is often the most comforting conclusion to those involved, other than the manufacturer. And, I wouldn't hold my breath for some determination from Mexico confirming such, and if stated otherwise, it would only be by others accepted with some skepticism. But it is possible!
More often than not (in my experience), manual transmission shifting complaints such as in this instance, prove to be clutch operation related, hence Tremecs' early concerns in this arena. One possibility, is that a relatively small rotational torque input is being applied, a sum that the other gears can, at least for now overcome, but that the 5th gear unit proves marginal in the task. One should understand that the synchronizer assemble for the 5th gear is the smallest, least effective as compared to the others, within this transmission. That understood, is the reason for my inquiry of how one established the "air-gap" value in my previous post.
Allow me to present a perhaps, simple-stupid test procedure, that seems to work most often: first, park the vehicle, engine off, on a prepared road/driveway surface with a slight incline, say 3° +/-, being held from rolling by transmission gear engagement only. Now slowly depress the clutch pedal, at what point in the pedal travel does the vehicle begin to roll if only slightly? Then continue to depress the clutch pedal, the vehicle should accelerate in its' rolling, now modulate the pedal finding the travel positioning that allows one to effect the rolling rate, above this the vehicle stops, below this there is no change in the rolling speed, this rage of pedal movement falls where in the available pedal travel? There should be a travel sum both above and below the effected range before the carpet.
Next, on flat ground, start the engine, allow a reasonable idle speed, at normal operating temperature, depress clutch pedal, place gear selector into any synchronized gear position, then place selector into the neutral position still holding the clutch pedal depressed, pause for five seconds, engage reverse. Does the reverse gear engage without grinding? Sometimes, particularly with beat-up gear sets (reverse) the gears will "blunt" and not allow engagement, but also not create any grinding, this is not our concern here, just try again. What one is looking for, is by engaging any other gear in the box with the vehicle not moving you have halted any rotation of the clutch disc, input shaft, gear sets, etc., and maintaining the clutch pedal depressed and with the pause in neutral, the observation is whether these components are re-accelerated, therefore indicating an input value as the reverse gear is not synchronized.
Also, there is driver technique to explore, but this is best accomplished with direct observation, and at times with care, as humans can be emotionally sensitive; but one topic we can preview, as pointed out previously, the 5th gear synchro is the weakest (and so is the gear set), and the differential in component speed is significant, so remember, the race is over once one chooses O.D., no speed shifting into O.D., it won't survive such abuse, allowing the R.P.M.s to drop in the execution of the shift is just good practice, besides you just traded the leverage advantage to the rear axle.
Scott.