[elmariachi]

Is it a grinding, or could it be a vibration? I experienced these exact symptoms shortly after getting my car on the road at 1300 miles and it turned out to be a sloppy pilot bushing transmitting a gritty, course vibration through the shifter. As I rev'd up through the top of first and prepared to shift, it felt like gears were grinding. Even though we dialed the Lakewood in twice with two sets of eyes, it was clearly not in spec. When we pulled the tranny to repair the pan leak at 3k miles, we found a badly worn pilot bushing. So we dialed the Lakewood in AGAIN and replaced the bushing and all was fine instantly. I just passed 4k miles and although I don't have the gritty vibration again yet, I can tell the pilot bushing is getting sloppy again because of how it grabs and pulsates sometimes in reverse. I am convinced the crappy tolerances of the Lakewood and our inability to get it dialed in is wallowing out the bushing, so the Lakewood is going in the dumpster in favor of a Quicktime at some point. Sorry I can't offer more advice.

BTW, which bellhousing are you using?