I specify bronze guides in every head buildup.

As mentioned in the above post, the #1 cause of new engine destruction is bronze EXHAUST guides. All nicely detailed in the above post. As a machinist and engine builder I have seen it a hundred times. I use cast iron exhaust guides except in aluminum heads where bronze can survive. Smokey Yunick once wrote that exotic valve guides should be put where the sun doesn't shine.

So lets start over. bronze guides? Why?
There is absolutely nothing wrong with a cast iron guide and it is absolutely trouble free.

You can replace your exhaust guides with bronze but to get them to survive in a racing situation you must clearance them to the point that the guide is as lose or loser than the worn guide you replaced (.00275 to .003"), in which case you have to ask yourself, Why am I doing this? Otherwise you will make one or two passes and then on the next one the ex. valve will more often than not hang up in the bronze guide for a split second. Sometimes like above you get lucky. Sometimes the motor goes. I absolutely refuse to use them in a cast iron head.
They work much better in bench racing than real life and there is absolutely no good reason to use them except that they can be installed without a proper guide machine. In other words they are a cheap Mickey Mouse repair. Even solid guides can give a lot of trouble in the exhaust in a severe service situation. I love it though when my competition uses them! ;>)

If you use them and are extremely lucky you might get away with it, but maybe not. If you use cast iron you will have absolutely no trouble. The choice is yours. Maybe or trouble free. I have made my choice. SBI cast iron guides. Quality and reliability.
Don