Im pretty certain your problem lies in how you reassembled the shift fork rod assembly on the top cover. I think you need to dissassemble back down to where you can remove the top cover again and check a few things (probably not what you want to hear but youve had enough practice at it now!). If i had to guess Id say theres a chance the roll pin in the shift fork rod isnt in far enough or is in too far and thats catching and not allowing you to grab gears. You have to make sure the flat spot on the shift shaft rod is facing you inside the top cover. Id double check the orientation of the horseshoe and the 1/2 and 3/4 inserts on the horseshoe. With the cover off and on the bench, move the shift shaft with your hand, rotate it etc as if you were going through the gears and see if you bind up as it tries to go into the gears youd be shooting for if you were driving down the street. Make sure the shift forks are oriented properly on the shaft. The downward angled sides should both be facing the rear of the tranny. If you want to take pics and or video of the shift fork cover assembly and post them Ill look at it and see if its right. If you can move the sliders into the gears by hand you should have no problem when using the shifter handle. And be one hundred percent sure the forks are resting in the valleys of the sliders when assembled.