Good nerdery. P-51 has higher speed and ceiling per Wikipedia. I assume that the P-51 performance specs are at least for an aircraft with 6x.50 installed. Unsure if it's for a full load of ammo (doubtful). T-6 specs are for an aircraft without any weaponry. Adding any will result in a performance decrease regardless of how the weapons are installed. At the very least the P-51 could stay high and zoom down on the T-6 and then escape with superior speed. So I agree with
@sevenhelmet, a P-51 would win due to performance edge.
All this obviously assumes you manage to weaponise a T-6B. Because the question asked a T-6B I assume the OP meant "stock". Because otherwise the P-51s going to win unless the T-6 driver is a kamikaze.