Ok, so I tried a new method and it seems bulletproof!
1. Warm up the aluminum (distributor housing or carb float bowl) with a small torch, just enough to make it hot to the touch (like your hot tap water).
2. Spray with Kroil penetrating lube. The heat allows it to soak in quickly.
3. Weld a dab or metal onto the tip of the broken screw (MIG or TIG.)
4. Spray again with Kroil.
5. Grab that bit of weld with a pliers and unscrew it.
Its just that fast and that easy. So far I'm 3 for 3 with no damage to any castings and each screw came out very easily!!! It'll probably work for broken head studs, thermostat studs, etc... but I haven't tried them yet.
EDIT: Sorry I meant to postthis in the MGB forum, but I guess it works here too!