You should spend more time justifying this set over Excadrill, which is faster, more powerful, packs the same defensive typing, and fits Protect into its set more easily. Body Press and bulk are both nice, but the former overlaps plenty with Ground STAB and, per the below, the latter does not substitute for speed on offensive mons.
You are still misunderstanding Body Press. Its base power of 80 does not change and has nothing to do with the user's base stats. When damage is calculated for Body Press, the user's Defence stat (i.e. the real stat, not the base stat) is used in place of its Attack stat.
Low speed can force you to take an attack from your opponent before you (hypothetically) KO them back. If your Pokemon is fast, then that doesn't happen. So, high bulk is not always an appropriate substitute for high speed on offensive Pokemon -- and this is before we consider the calcs you asked for:
* 252+ SpA Life Orb Primarina Hydro Cannon vs. 96 HP / 160 SpD Steelix: 788-928 (250.1 - 294.6%) -- guaranteed OHKO
* 252+ Atk Choice Band Dracovish Fishious Rend (170 BP) vs. 96 HP / 0 Def Steelix: 386-456 (122.5 - 144.7%) -- guaranteed OHKO
* 236+ SpA Life Orb Lapras Hydro Pump vs. 96 HP / 160 SpD Assault Vest Steelix: 291-346 (92.3 - 109.8%) -- 56.3% chance to OHKO
* 252+ SpA Life Orb Kingdra Muddy Water vs. 96 HP / 160 SpD Assault Vest Steelix in Rain: 390-460 (123.8 - 146%) -- guaranteed OHKO (*without second target)
* 252 Atk Life Orb Urshifu-Rapid-Strike Close Combat vs. 96 HP / 0 Def Steelix: 276-328 (87.6 - 104.1%) -- 25% chance to OHKO
* 252 Atk Life Orb Terrakion Close Combat vs. 96 HP / 0 Def Steelix: 276-328 (87.6 - 104.1%) -- 25% chance to OHKO
* 252+ SpA Charcoal Torkoal Eruption (150 BP) vs. 96 HP / 160 SpD Assault Vest Steelix in Sun: 558-656 (177.1 - 208.2%) -- guaranteed OHKO
* many, many Pokemon that can 2HKO Steelix (which are relevant to the argument, because if Steelix were faster and less bulky, then they would lose the 1v1).
The idea behind the set is nice, but it is more niche than you think it is. It would be interesting if you could show Steelix's bulk making a difference in high-rating ladder games.