Generally when there is to much slack in the cable and the clutch is not fully disengaging , the trans will be hard to shift into first gear when sitting still or any gear while driving or starting out. You can push the pedal to the floor with one hand and with a large screwdriver wedge the quadrant so it stays in place then lift up on the pedal and you should hear it click a few times. This will set your tension a little tighter on the cable and allow for the clutch to better disengage.
How ever if this does not rectify your grinding problem , then I suggest you contact who ever rebuilt the trans. Im assuming this is a T-45 trans? Misplacing 1 little spacer in this trans can cause all kinds of shifting problems. There is 0 tolerance for mistakes on these trans ( or any trans ) when rebuilding them. I recently just opened up a t45 that was having trouble with 5th and reverse, and also the trans was clunking between gears. I found that who ever pulled it apart last left out a very thin spacer behind the input shaft bearing cap. Just that .030" spacer caused the whole assembly to be off , and caused a lot of slop in the main shaft and input shaft.