dollar for dollar, the 302 will beat out a 200 and 250 for the price per HP.
Pound for Pound, the 302 will beat out the 200 and 250 for weight per HP.
Sure, there are other things that have to be done to convert, but still...the overall advantage will go to a 302 everytime. the 200 does not save enough weight to compensate.
Anyone who says otherwise is smoking something. There is a reason why the 289/302 block lasted for over 40 years in Ford's line up. Explorers had them until 2001. The amount of aftermarket parts for the 302 makes the price so cheap it's ridiculous. for the sixes, we used to have Clifford. Now we have Mike (who does keep his prices reasonable for the market) and our own innovation.
Building a six is not the same mindset as building a 302. If you want lots of HP. 302. If you wants lots of well documented builds? 302. If you want cheap HP? If you want to look like every other resto-mod mustang at a show? 302.
If you want to be slightly different? Build a 6.
I met a friend once at a show and shine. He brought his beautiful 68GT500. Gorgeous car. He parked beside my 65. Now, the pictures of my engine bay make it look better than it looks in person. It is rough. The car is now where near showroom or show condition. I had a bigger crowd around my car than he did. Why? Everyone has seen built V-8s. So little out there that hasn't been done already at least a few hundred times. How many people have seen a tri-carb 200? (which is what I was running at the time).
. Honestly the mod doesn't look too hard and i'm seriously contemplating attempting it with my buddy in his garage and maybe even making our own carb plates. 
