Given that choices may be limited in terms of the available manifolds, perhaps the best approach might be to get a custom gasket made that doesn't overlap the ports in the head but does provide as much sealing as possible around the new manifold.
My very limited understanding of gas flow suggests that the effect of the expansion into the larger diameter of the manifold pipes will cause a reduction in gas velocity but hopefully not too much unwanted turbulence so long as the port edges aren't interfered with by the gasket?
Certainly not ideal if one is looking for ultimate power, but if one is not looking to race the car one could probably compensate by simply dropping a gear and pressing the accelerator harder if one wanted a bit more oomph in any given situation
