374 are the best castings for porting next the 643's. The 374's have cleaner exhaust ports and somewhat larger intake with thicker walls for porting. The 643 were supposedly revised but the pair I ported had lots of imperfections in the exhuast ports though the intakes were nice. The 561 heads are thinner and have the best flow for stock heads. You can get cleaner castings in all numbers. Look at the heads for consistent runner size and less clumps bumps lumps in the intake and exhaust.
I've got 3 sets. I know one set has the 643 casting. The other two are still in the car and one set is crated up. I run in a race class that doesn't let you mess with the heads. So, I thought I'd sell one set and keep one for a back-up. I think I'll take a look at the other two. Maybe one will be the 374 casting. Thanks for the good info.
Try to locate a set of 561 for limited sportsman class racing lighter castings better flowing. And ya know soda blasting the insides of the pockets to the ports to make sure their good and clean isn't considered porting he he he.
Lloyd Elliot flow tested them in stock form vs 643 and 374's post #20