A splicing block assembly for splicing a length of leader tape connected to a hub of a cassette to a length of magnetic tape while the cassette is in a cassette load which includes a splice block mounted for reciprocating lateral movement between a first position in tape alignment with a first cassette position and a second position in tape alignment with a second cassette position. The splice block has a planar splicing surface segment having at least one tape receiving elongate track therein. A shift block is mounted on the splice block. The shift block has a planar splicing surface segment flush with the planar splicing surface of the splice block with first and second tape receiving tracks therein. The shift block is mounted on the splice block for reciprocating lateral movement between a first position where the first track is in tape alignment with the track of the splice block and a second position where the second track is in tape alignment with the track of the splice block. The splicing block is mounted on a vertical front panel of the cassette loader and reciprocates in a substantially horizonal direction.