I reckon you could definitely do 600km. You can go faster in these things than in audaxes, because the courses are faster, all your food is to hand, you don't have to faff about at controls and you get that extra bit of speed from the magic numbers on your back.
Training: you've got the distance nailed already. Work on your speed.
Position: It's hard to say without seeing what your current one's like! Generally with tribars, the idea is to move your saddle up and forward and your bars down. For a 24, people generally then move their bars back up a bit again, and possibly put a slightly shorter stem on. If you're going to use tribars, you need to get them soon and then train in them lots, otherwise you won't use them after the first afternoon on the 24.
Planning: Get a plan well worked out so that you don't waste time faffing about on the day. Think about whether you're doing it supported or unsupported and work out how much food you'll want to take on and where you'll want to dump it if unsupported.
Read some stuff by other people who've done it before. I've got an article for the club mag from this year which I'll stick on here presently, but for now,
here is something from a fellow first timer from my club. Oranj has got a report up somewhere, either on here or on his blog. There's then loads of stuff in the TTF archive, I particularly like the report on
Roy Cromack's ride.
http://www.youtube.com/v/Z9g_g73Iqeg&rel=1 with Andy Wilkinson is also really informative as to how he managed his massive distance in 2011 (apart from being a robot of course.)
I've said it before, but enter a 100 to give yourself an idea of what long distance time trialling feels like, and to test your setup.
Here is last year's 100 calendar - I'm guessing the Hounslow event would be most convenient for you. Note that it didn't fill up (max is 120) so you should have no problem getting in.
Oh, and enjoy it!