[Steeveedee]

I didn't watch the videos, so no comment there. You probably have no synchro for first if it is the original trans, so it's going to grind on a downshift. Wait until you are completely stopped before shifting to low (truck still in 2nd or 3rd gear, clutch disengaged). Check your clutch free pedal, and if that's OK, pull the trans into 2nd or 3rd before going to reverse or 1st, as that will stop the cluster gear from spinning, and you will be able to put it in those gears with no clashing.