We further develop the massive constructive theory of the Standard Model and use it to calculate the amplitude and squared amplitude for all 2-body decays, a collection of weak 3-body decays as well as Higgs decay to four neutrinos. We compare our results with those from Feynman diagrams and find complete agreement. We show that in all the cases considered here, the amplitudes of massive constructive theories are significantly simpler than those resulting from Feynman diagrams.In fact, a naive counting of the number of calculations required for a matrix-element generator to compute a phase-space point is orders-of-magnitude smaller for the result coming from the constructive method suggesting that these generators might benefit from this method in the future even in the case of massive weak amplitudes. *