You're leaving the roof over the porch I take it? I'd wedge some 2x4's nice and tight between the roof and the floor before you do anything. 2 near the rear on either side should do the trick. Leave yourself enough room to work around them of course. Mark all your lines first, dont forget your gloves and eye protection, and go to work. Angle grinders aren't terribly difficult to use, just go slow, use good discs and pay attention. They have a lot of torque and will slice through a finger with little trouble. Its not terribly difficult, just respect it like any other power tool.
http://www.skoolie.net/forums/showthread.php?t=9883 - 7.3L 4x4 Shuttle Bus Toyhauler conversion