MPG build 360
Magnum head ports on a 360 will keep velocity up in low rpms just fine, but give you a bunch more on the top end. Mileage would also be the same or maybe even better than the 302 heads.
Flow comparison is about 155cfm @.500 lift for 302, and about 210 cfm at .500 with the magnum head with factory ports and valve jobs on both.