I've asked this before and got no response.

My spring pockets are 1.550" in diameter, and the springs I currently have have 1.254" outer diameters (comp 981 springs). My heads are iron. Would having that much clearance hurt anything? If so, what would be a good safety margin for such a clearance?

I'm planning on upgrading to the crane 96808-16 springs, and they have a 1.514 outer diameter, would that be an okay clearance?

I know that with aluminum heads this would be unacceptable... what about with iron?

I've never seen this discussed here and can't find any information in the web or in my books. I'm about to order the rest of my valvetrain and need to know what to get... please help!

It is best to have locators to keep the spring from moving around just as retainers should fit the spring properly.
If you do not have room for a cup use a locator. It looks like a spring shim with a lip on the inside to locate the spring from the inside