Bread Flour vs. AP

Toast

Making a 75% hydration loaf--slightly modifying the Tartine Bread recipe. What will be the effect of substituting bread flour for AP flour?

...is that the loaf will be a bit more chewy, both crumb and crust, but otherwise not much.  I prefer the bread flour option.

The bread flour will absorb more liquid and for the same weight of flour will produce a stiffer dough (which will mix differently, shape differently, and slash differently). You might hold out a few percent of the flour until you are through autolyse and see how it feels/looks. You can always add it back if you need it.