mmk, after seeing everyone taking their board covers off, cutting up their chassis, and god knows what else, i did it , and i didnt have to do anything extreme at all...
No pics of the procedure, but i do have pics of it finished...

Materials Needed:
Xmods Evo
Cyndrical Cell Lithium Kit w/ Board Harness and 4 Zip Ties
Soldering Iron
Flathead and Phillips Head screwdriver
Wire Strippers
Side Cutters/ Nippy Cutters
Flat File
Drill Set/Dremel Drill
Dremel(optional)

1. Take the screws off your ep cover, unsolder the power wires.

2. Take the screws off your battery case on the bottom of the chassis, use a flathead or small screw driver to pry the case up and off the main chassis.

3. Cut the little tabs off, then file them down smooth. Stick your zip ties through the slots beside where the tabs used to be and pull them through.

4. Take the lith kit, and zip-tie them to the chassis(the cells should be to the outer part of the chassis, not on the inside). Tighten the zip-ties as much as you can,cut the ends off, and make sure they are clear of the screw holes for the ep cover.

5.Cut/Snip/Dremel out the center peice of the battery case, the part with the screw holes. Screw it into place and make sure your xmod is in the off position and label the on and off sides with a sharpie or something.

6.Screw your board back down, and break out your soldering iron and if the kit has a Gen1 No Drill harness, cut the pins off and strip the wires, then tin the wire with solder.

7. Solder your board harness to the board.

8. Drill a hole big enough to stick a dean plug through in your ep cover, thread the dean plug+wire through, screw down the ep cover.

9. Re-install your antenna (*hint:heatshrink your antenna), crystal, and motor wires.

10. Plug your liths up, make sure everything is working right, and go have some fun with some 7.4 volt power..haha

Pics of it done:

^The only reason that screw is out is because i stripped it earlier, if its not stripped it will screw right down...
:)