Khan,
Having just helped a friend with this, I can tell you that access from the bottom is pretty good.
The advice above is all good. The splash shields should come off and having the shocks and springs out (as a unit) makes life a bit easier too, although not essential. There were (on my friend's car) a couple bolts that were very tight access (at the front of the engine) and having run the new bolts in and out of the holes (in the heads) a few times makes it much easier to finger-tighten them, saving tight-access wrench time. Obviously, as Marlin says, get all the bolts in a few threads before tightening any of them.
Mark
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