How would NT faith differ from OT faith?
No difference-one faith, one gospel
If a) faith is in God and b) Jesus is God, the only difference would be the quantity of knowledge to which we assent, not a different object of faith.
Gal 3:8 And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed.
The same faith that was proclaimed in Gen 3:15 is the same gospel we have. One cannot be saved if their belief is monotheistic, i.e., Judaism.