The best 4E conversion is the "RPGA" one. The book is something else.
The Dungeon article mentioned is also good, probably the best source.
Some conversions do touch on the 2E "return to" material, but its very easy to ignore that, impact is very small.
The original is your touchstone in any case...the thing about it is that it is so focused on player choice, versus mechanical issues, that it could be very easy to convert. Maybe too easy.
If you want deadly, set some default DCs. Have things like finding secret stuff work as per the module, but always assume minimal impact on the PCs part. Monsters require some conversion, though things like damage are almost in the ball park as is. When something kills you in the module, well, it kills you.
Think through how resting, ritual casting, and spells that can allow escape or bring aid work (this is very important with high level parties). The original module had some restrictions, including a demon gating in under certain circumstances.
Now, if you want to tone certain things down and be a little more formal in converting things, take a look at the 4E RPGA module and the recent dungeon one (and the 3E one doesn't hurt).
You can also just have some rules of thumb, instead of auto death, drop to 0 hp, for example, where appropriate.
Finally, for the big little skull, don't overdo it. The point is that it is deadly in a very horrible and straight forward way. Try to stay in the spirit of that, even if you don't want it to kill the whole party. Again, take a look at the conversions for this, though some of them add a little too much zazz at the cost of the real horror.
