Ok so this season, prolly back in june, i blow up a transmission (that was obviously on its way out) while shifting into 3rd. I take the thing to a very reputable builder who takes the thing apart and upon seeing the utter destruction that was my transmission, he starts fresh by rebuilding a tranny off a donor. Sure enough my new transmission with all fresh parts makes a healthly grind at high rpm going into 3rd gear ESPECIALLY on downshifts. long story short i take it back, synchro damaged, new build, and still more gind. Take it back, synchro damaged, new build with a brand new synchro and gear, and yet it still grinds!.

Now i know nothing about transmissions but the guy doing the work has a great reputation. What possible defects could be causing this grind? For reference I am a religious heel-toer, I have made sure that i have been getting the clutch all the way to the floor, i have tried double clutching but with no better results, and the only way i can get it not to grind on the downshift is when i move the shift lever very slow and not apply much force. -resulting in a much slower shift

Thanks for your help!