I'm no expert, but I'd look at what gasket you can use to achieve a good seal, especially on any water passages. The mis-match at the ports will tend to create some turbulence just inside the intake, which will lead towards a thorough mixing of the air/fuel charge and might even promote a more even mix across the cylinders. Unless you're looking at trying to eke out the last couple of horsepower or ft-lbs of torque I don't see that it will be a problem provided you can get them to seal up fully. Others who are more expert on the BBC may be able to point you to specific gaskets, or may say that it simply cannot be sealed.