Yeh once they are all the right tempo, cut the start of the tune on exactly the first beat (if you don't accumlative errors will lead to stuff flamming/clanging), then use a real decent filter to switch between tunes. Its all about careful EQ automation of the highs and lows to make things sit right.
i started one - TOO LONG.. So I just did it on 4 decks ha! Be prepared for a shit load of ball ache, it takes a long time if you want it sound "real"!
on logic you can Latch things so to find out what type of automation curve you need, I set up the hipass filter to one of the knobs on my midi keyboard and mixed the tunes live so to speak. Then re-drew the automation curves neatly afterwards.