Kinetic Monte Carlo simulations have been used to investigate mechanisms for boron clustering in crystalline and preamorphized Si. We have extended previous boron-interstitial cluster models to include larger and more stable complexes in order to reproduce boron cluster evolution at very high boron concentrations. We have investigated the stoichiometry of boron-interstitial clusters resulting from low temperature recrystallization of preamorphized layers. We have performed a dedicated experiment based on boron implanted into preamorphized Si with end-of-range defects placed far enough from the boron profile to avoid the interaction between end-of-range defects and resulting boron-interstitial clusters after recrystallization. Hall measurements on active B dose combined with a systematic analysis performed by Kinetic Monte Carlo simulations indicate that initial boron-interstitial clusters after recrystallization should not contain a high amount of Si interstitials. Otherwise, boron deactivation and subsequent reactivation will occur faster than experimentally observed. The present results suggest B3 and B3I clusters as the most probable configurations after recrystallization.