i doubt theres a set formula when it comes down to it.
its safe to assume that 3.8 top swap is like 3.4-3.6 l67.
your ported heads will really help with the flow. because of that, you can probably even drop to a 3.6 on your top swap setup, but the only way to be sure of that is to throw the 3.8 on there and see how that runs.