I have had some factory heads that were close to max effort done by some of best around in my area. gerald geary, indy cylinder,herb mccandless etc. they have spent countless hours on the flowbench trying to get every last bit out of these heads and the best set i have had flowed 310 i think and they kept cracking and went to the scrapper after being welded many times. they were ran on a pulling truck the last few years until they cracked worse. these guys have spent 40+ years porting these heads and they could never get those numbers and the head still be usable.