Joe, the r302 is iron, dart makes both an iron and an aluminum block. Don't let your mouth hit your keyboard when you see the prices of it...
http://www.streetrodstuff.com/Products/1834/
Also, from what I've seen on the net, he sportsman block is not what it "cracked" up to be so to speak... there are a lot of guys cracking them, from what I've read on the net ford has actually decreased the HP ratings for it in the motorsport catalogue. I can't confirm this, but I rea about it a lot. Do a search on the power adder forums, and turbomustangs.com and you'll see what I've been reading.