Here’s a “YMMV” story......

One of my customers had a very low use set that had zero porting, and a fresh VJ done at Barton’s for 1.81/2.14 valves, along with 16 newly installed bronze guides.

They wanted to put them on a FAST stroker build.
I ported them, they put the motor together and ran it.

The car ran pretty good, but after a handful of trips to the track they started getting nervous about not using the correct heads, so they had me do some 906’s to replace them.
They weren’t happy with the idea of losing power with the head swap, but iiwii.

They brought the motor here and we did the head swap on the dyno.
Both sets of heads had the same valves and springs...... both flowed 280cfm.
My recollection is the 906’s made about 30hp [i]more[i/] than the stage iv’s on that particular motor, and the car ran a bit quicker with them too.