spray it with PB Blaster or liquid wrench. Let it set overnight. You will need a large breaker bar to get enough leverage on it.
Keep at it, you'll get it and the Mac Pro-Chamber is a great mod.
4.10's in an 03 automatic driving in OD at 70 mph. 4.30 would be a bit much unless you are really into racing and don't drive it on the streets that much.
I'm running a PA Racing Tubular K-member with stock a-arms and Eibach Pro-Kit spings on my Daily Driver. Handles just as good as the stock K.
I get a little more engine vibration from the stiffness of the PA K.
Overall I think its a good product.
Here's a link on how I did the install...
If you take the K-member out, for less than $400 you can put a lighter one back in. Its a lot easier to work with and saves a lot of weight. Here's a pic of a PA Racing K-member I put in when I did BBK headers.
Any time you remove the K-memeber, you need to have the alignment checked.
Here's a good article on installing headers by dropping the k-memeber
http://stangfreak.com/content/index.php?option=com_content&task=view&id=52&Itemid=31