Small port heads, big port intake. How bad?

For a street grocery getter your probably not going to see much improvement by going with a small port intake. But with that said you are loosing some low RPM torque, which will hurt fuel economy some. If you were using this on a truck that was towing I'd say make the change. As for the carb you are using that should work quite well with the 318 though they are normally jetted for motors in the 350ci size so dropping down one jet size or going up one rod size will also help. Try to take it to someone who can load test it either with a dyno or at least with an O2 sensor and doing a brake stand (redneck version).