Welcome Ponyrider66:
You have come to the right place for advise. Many wise and experienced 6 cyl owners in here.
Anyway, about your problem.
Ashame the head broke. I know the bolt you are talking about, I have broken many of those. Usually the bolt breaks and you can bore/press it out. However since the head cracked and gave way you are going to have to replace it. There is no known way to fix it that will be permanent.
In replacing the head, you have a few options. First tell us more about your engine. Is it stock? What year are the block and head currently? What carb and distributor are you running.
If it was stock and you wanted to maintain that, you would start looking for a 200 head from around the same time, 65-68. Most of these will be log type heads with 1.5 carb openings and the smaller valve sizes.
This would probably replace your head if it fits the stock year. I would have a three angle valve job done, have hardened seats put in and you could just swap them out.
If you want to upgrade your engine a little. You could do one of many different things....
Replace the 200 head with a US 250 head from a later year and get a larger carb openning and valve size. Match this with a set of headers and either a larger cfm 1bl carb or use an adapter and mount a 2bl carb. This will give you a little boost depending on how much you do to the head.
You could go with the above plan and find a "M" flat-log head, have the intake modified to accept a 2bl carb. and follow the rest of the above plan.
There are other options if you are willing to go out of the US for parts. There is a very feasable Aussie 2v head with seperate intake that is fabulous. And there is also an Argentine head although I think it is rare and hard to get ahold of.
As for parts search online for Mustang Parts, Vintage Mustang Parts, etc... you will turn up dozens of mail order and web based parts suppliers.
Some of the better ones are:
www.mustangsplus.com
www.mustangsunlimited.com
www.californiamustang.com
www.metromustang.com
www.sumittracing.com (Not a lot of 6 cyl parts, but the best dealer anywhere!!!!! I buy from them anytime I can.)
Those are my main dealers. I have gotten everything I have ever put in my Mustang from one of them or local yards and shops.
A good source for the different head options and ideas in general is Mustang Geezers site:
http://www.geocities.com/mustang_man_1966/index.html