70(Base Power) X 1.5(STAB) X 1.5(Guts) = 157.5
STAB is also taken in to account when Guts is active, the only time it isn't with offensive moves is for set damage moves(example Seismic Toss),so a move with a base power of 70 with STAB + Guts would have a base power of 157.
Source: Experience and math.