Okay, I know what I['ve used in the past, to modify aluminum. I had to hog out the centerholes on the wheels I have my snowtires on and it was, um, tedious. So, let's just say if a feller were to find hisself needing to, say for instance, do a lot of grinding and reforming to a bellhousing, what manner of tools would be best to use, this side of a nice big milling machine?
If you're talking handheld...
A die grinder with a burr (I guess that's what you call it...not a stone) and a helper to spray wd40 on the area being worked.
WD40 works magic while grinding on aluminum in my experience.
Clem
A not-so-nice or not-so-big mill? 
If going hand-held, minimize the hand-holding. Clamp/bolt the workpiece down, provide tool and/or hand rests, etc.
Don't forget the face shields for everyone involved. WD40 and aluminum chips are no fun in the eyes.
Can you give us an idea of how much you have to remove and what shapes you're dealing with?
I'll third the die grinder and carbide burr. Anything else is a waste of time. And for the die grinder, don't go with a HF grinder, use a name brand unit and you won't kill it. FWIW, I used to port and polish heads (still do every now and then) and I created a down-draft booth to keeps most of the chips where I wanted them. I used a very powerful shop vac that was pulling through some ducting. The head was sitting on a grate, below the grate was a furnace filter, below that was the ducting running to the vac. If done right, 75% of the shavings will go in the filter, the other 25% will somehow find thier way into your plumbers crack.
Ah, WD40... I'll give that a try. That and a better diegrinder than the air thingie I have here. As for what needs doing, I have to grind away here and there on a Toyota Alltrac bellhousing to make it so it'll talk to my BPT Mazda block/adapter plate, in an attempt to build some reliability into my rally car. Mazda gave us a great engine and bolted it to a truly weak awd trans, damn their eyes...
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