You can't polish cast iron parts. Sorry man. And they'd instantly rust if you did, like the first time you start it.

If you want a sexy finish for those, www.jet-hot.com will do it for around $100. They bake on an extremely hi-temp finish that actually lowers underhood temps by up to 40 degrees, and makes the manifold itself last longer. It's available in finishes that appear to be chrome, but the best coatings are the ultra-high temp coatings. You may want something that can handle more than 1600°F, and I think that's as high as the chrome-looking finishes go. They do have a sexy gloss black that holds 2000°. They can do your turbo's exhaust and o2 housings, too. Get a quote.