I've been running big block mopar for all most 40 years the problem there's very little RD I think the hemi got all the Rd in the early years I worked a machine shop for 15 years I cut up more 906 and 915 heads to look at core shift my 67 gtx with 512 with steel heads went 12.20 and drove it every were B1s a good set flow 400 in the intake I ran B1s up to 5 years ago they haven't changed that much my 588 with B1s from best made 1050 hp and went 8.20 at 2800 pounds I now run a set of heavily port Indy 600-13 XXX head with the help of Mike Sanders Chuck from Best and Tod Goodwin these heads flow over 500 on the intake and 390 on the exhaust we are still learning this combo the motor has made well over 1200 hp and 900 ft pounds of torque we raised the in and the ex a 1/2 inch cost come into a factor I've been very lucky to get the help to keep racing Mr Sanders and Best Machine can't say enough