727 works fine but wont manually shift 1st

I had that issue when I put the big block in my Demon. The the arm at the trans on the torque shaft was hitting a boss on the bell housing. I ended up cutting the welds on that arm, re indexing it and welded it back to correct it. I would first adjust the linkages to make sure they're correct then start looking for interference somewhere. Could also be parts that aren't matched properly. To isolate further you could disconnect the linkage at the trans shift lever, move it thru by hand making sure it hits all the detents and move the column shifter thru to see if it gives you full travel. Just make sure the linkage will clear everything while moving it. Make sure you chock the wheels so it doesn't roll on you.