"best" is all relative for what you want to do, like low end torque for the street or big turbo setup for top, low compression for a power adder end ect. After you decide what you need from the motor, then you can decide on what to get and how to fit it.
rpm-mustangs . com sells engines that may be worth lookin at. They have 3.8s and 4.2 strokers with or without forged internals. If i were going to turn my sixxer into a big project, i would probably start here.