small block chevy head mismatch

i was wondering if these heads will run right together.

3986339

458642

they came with an engine i have and im not sure of the specifics of them...all i know is that they dont match and the valve stems from 3986339 are too small a diameter for the 458642....the valve heads are the same size....are the combustion chambers different sizes and if they are, how big of a difference are the two from each other (and am i going to notice it running rough)

Looks like a mismatch to me. Even though you measured the valve sizes the same - my database shows different. Maybe they were changed at the factory or somewhere down the line during a rebuild? Hard to say, not a exact science.

Anyways the chamber size shows different and also the intake port cc's

http://www.hotrodink.com/Images/data/546/1mellobud4.jpg

http://www.hotrodink.com/Images/data/546/1mellobud5.jpg