Hi Damon,
I don't have a specific cam recommendation for you, I think your best bet is to call comp cams tech line (which has a really good reputation) and ask them. They will want to know what RPM range and style of driving you are shooting for. I have built two smallblocks similar to what you are planning lately and kind of missed on the cam selection on both of them! I used the weiand Stealth intake instead of the RPM however, probably about the same. I read an article recently that showed the new RPM Air Gap as being probably the best intake available right now.
The first is a 302 that I used the same heads (GT-40P) as you on. I went with the ford motorsports B303 roller cam. Roller cams are pretty magic really, you might want to consider a retrofit one for your 289. This motor runs great and really winds up, but sometimes I wish it had a little smoother idle.
I also built a '67 289 recently with stock heads (stainless valves and a little exhaust port work), exhaust manifolds, and a crane 266H dual pattern cam. This motor has an automatic behind it and 3:00 gears in a heavy galaxie. In this case I wish I would have gone bigger on the cam. It could be the exhaust is corked up too. I put a AOD automatic in this car too if you are interested in any details of that.
Some other issues I would suggest you consider is that switching to GT-40P heads gave up about 5 cc of combustion chamber volume from stock. Is the 9.3:1 considering that? If you haven't yet, you should check the heads with your headers to make sure the modified spark plug angle isn't a problem with these heads. Mine wasn't an issue with the headers I used but I definitely could not have used the stock cast iron manifolds. Also you might want to consider a rear end gear in the 3.5:1 or higher range, 2.8:1 is pretty tall.
If I were building a '65 mustang I would keep an eye on the modifications done by Dallas Mustang for their Terlingua turnkey cars.
http://www.terlinguaracingteam.com/
Take Care, Chuck