more out of 5.9l 360 magnum

Yeah it's the Performer RPM, but it's generally just called the airgap, p#7577.
http://www.summitracing.com/parts/edl-7577

Fitment wise, it bolts onto the motor just like the other. The only real differences would be things like thermostat housing, coolant fitting locations, etc, but you can just compare a picture of the airgap and the one you have to see the differences. There should be no major differences unless the intake you have has the larger (older style) thermostat housing, but I'm thinking you have the smaller (magnum size) housing.

The best intake gasket kit I've seen (and have used several times on mine and other friends magnums) is the kit from Rock Auto. It comes with a few parts you don't need but for the price it's a good deal IMO.
http://www.rockauto.com/catalog/moreinfo.php?pk=2194496&cc=1361810

Here's a list of carbs I'd consider acceptable for your application. I hope this works, it's a refined list of carbs.
http://www.summitracing.com/search/...e=100&SortBy=DisplayPrice&SortOrder=Ascending

When it comes to the headers, 1 3/4" primaries will be more than plenty. No need for the added headache and expense of the 1 7/8". TTI and Dougs seem to be the ones guys prefer but I'd ask around. When it comes that there are guys that know more about the specific of installing headers in an Abody than me.

Exhaust wise, a basic dual 2.5" exhaust with either and H or X pipe and your choice of muffler would be fine.