Building my head right now and am undecided on which guide to use. I know the cast has tighter clearances. Any real advantage to using the bronze? building a strong street motor that won't see more than 4500.

The bronze guides last much longer then the cast iron.
There are many different types, use the solid (thick wall) ones if possible.
The thin wall sleeve type or the wound versions take more care to install so they don't slip / move over time.
DD

Are the bronze better all around, or are the cast better for a higher performance engine due to the tighter tolerances?

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'74 Duster w/ HEI ignition, beat to snot suspension, A904, 8.25" 3.55 SG rear, still being tuned up and gets 17 MPG

Know how they always build a better idiot? That's me

The race engines use bronze. (unless the rules say not to)
As for the clearances and tolerances guide to valve stem, the machinist sets that when he installs the guide.
DD

I do my own heads, and if you want tight clearances, you need good valve stems. They need to be round, straight and consistent size.

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'66 Cuda 225/4spd
'66 Dart GT convertible 225/auto
'64 Dart GT 340/4spd