Righteous Might and Enlarge Person will probably never fly to create Huge since they both contain clauses about size changes. Depending upon DM rulings on Polymorph (whether it is a size increase or a size change that sets your normal size to that of whatever is normal for the creature you change into) a Persistent Draconic Polymorph (Draconomicon) would be a great grappler.
Cleric 12 (or any combo of Cleric and PrCs that give you 12th level casting.) You need Spell domain to pick up Draconic Polymorph via Greater Anyspell.
12 = BAB with Divine Power
4 = Improved Grapple
8 (4) = huge size from Righteous Might and Draconic Polymorph (4 and Large if you consider Polymorph a size increase rather than change)
8 = Titan Gloves
21 (19) str mod = Str 53 or 49 (31 War Troll from Monster Manual 3 + 8 Draconic Polymorph +6 Divine Power + 8 Righteous Might (or only +4 if errata)
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53 (47) grapple mod.
There might be better forms with higher strength, but I like the War Troll. Dire Snake from Monster Manual 2 (or just Giant Constrictor Snake from Monster Manual 1) might be interesting choices. Huge Size and Constrict (+Poison with the Dire Snake.) You might also combine Holy Transformation, Lesser and Draconic Polymorph to get some nice Outsider forms.
You'd probably want to get off more than 1 Divine Metamagic Persist to avoid the tedious buff period and short duration of many of the spells. With the feat intensity of the build you might want to worship an ideal so you can pick up Planning and Undeath for free Extend Spell and Extra Turning. You'll probably want to be human as well. *Extend, Persist, DM

ersist, *Extra Turning, Improved Unarmed Strike, Improved Grapple, maybe Extra Turning x2 or something.