Have you done this before? You better practise on the cracked one first, you'll see how easy it is... to break em
You can't really cut the bodywork, pillars etc as you've still got to release the screen along the bottom, which is by far the most difficult. The blade on my thing wasn't really long enough, ditto a Stanley knife, tried using that from inside but would need all the dash out, hacksaw blade not strong/flexible enough, ideal would be a sharp bread knife or of course, wire.
Eventually managed by inserting only the tip of the blade in the Stanley knife, so it was effectively a half-inch longer, counted my fingers when I'd finished
The mastic stuff will need to be a continuous bead as I imagine it'll be near impossible to find, let alone seal, a leak; and there's not much time to do it, think the car can be driven after 2 hours!
This is what I've got , http://www.ebay.co.uk/itm/3209045866...84.m1439.l2649
think one tube might be enough but thought it better to have 2 to be on the safe side, I've got a couple of boots needing heels glued on if I've any left over