I just had this problem fixed on my 2000 F150 with the 5.4 engine last week. For the last 2 years I have put off fixing it myself and I found a local guy that fixed it for 125 dollars. I bought a new manifold from Rock Auto for a around 75 bucks and some stanless bolts on ebay for about 15 bucks. I got lucky finding somebody to fix it for that cheap and if you look around you may also. My manifold was rusted up bad also.
They seldom "break" off entirely flush with the head (not the block). The gaskets are a laminated steel, and once corrosion gets in the layers, you get a leak. You can't fit the new one in there anyway, without removing or at least loosening the manifold to pull it away from the head. You may as well replace all the studs, as Dan suggested, and all new gaskets.
The studs actually come out better than you would think, as they corroded off rather than broke off from too much torque. The biggest problem encountered is the bolts with the nuts still attached have corroded enough that they often end up a ******* size. I've routinely had to select a socket that is a fraction too small, and drive it on to remove the nut, or sometimes the nut and the stud.
The gaskets and studs/nuts from the dealer are fairly expensive ( and obviously they do not hold up). You can't go too far wrong by just picking up some grade 8 metric bolts/small washers, and aftermarket gaskets, like Felpro.
For easier access to some of the bolts, the inner fender may have to come out. You'll see why after spending a couple of hours on 1-2 bolts. I've always cut the down pipe bolts also, and just replaced them. Without doing that, moving the manifold around much will be limited.
Before you put the manifold back on, check it really well for hairline cracks.
I think I've replaced bolts/gaskets on maybe a dozen manifolds so far, and I'd say out of that many, I've only had to drill out approx. 6-8. Often as not, enough is hanging out when the manifold is off, that a small (5-6") Visegrips will twist them out. With the nuts corroded off, the torque on the threads is gone. Minor corrosion is all that usually holds the leftovers in the head.
If you get a new manifold, and NOT from Ford, it will likely be Chinese, so pick your poison.
You can find metric stainless steel bolts at ACE hardware. I believe "Stage 8" makes a SS stud kit, call them and see if they still do? If replacing the exhaust manifolds, it would be wise to just install some JBA SS shorty headers instead of wasting money on stock exhaust manifolds.
When removing the rusted exhaust manifold, don't bother trying to use a socket wrench on the nuts. Just get out the die grinder and buzz them all off. You can then just pull off the old manifold and as stated above, use some penatrent and a vise grips to carefully remove them. If some or all break off, you'll have to drill them out slowly with a cobalt bit and stud remover. Tap them all first with a hammer to help break the corrosion bond before backing them out.
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