I've raced many things with both iron and alum blocks--If weight savings and car balance is primary--balance the lbs vs cost per pound against other areas that you can achieve weight reduction--smaller radiator/
oil cooler/plumbing--brake system components--lighter exhaust system--suspension/steering system,etc. You can make the power level your looking for with a four cylinder so consider your needs/wants carefully---or DO IT ALL!!!!
Jerry