Sounds like a fun project.
If you're really trying to make a Maxx "look" like a 1/8 buggy you'll need a buggy body (maybe the XTM XLB 1/7 body would fit better ->
http://www.hobbypeople.net/gallery/149950.asp), OFNA 17mm adapters (
http://www.ofna.com/tires-monster.html (scroll down)), 1/8 buggy wheels, and of course the 1/8 buggy spoiler.
I'd get the g-maxx extended chassis (for more 1/8th authenticity) and probably 12-14 cells. NiMH will work but of course LiPo is better (read: lighter). You can probably run a 2800XL w/ taller than stock gearing (1/8 wheels are small and light).
Use a soft spring to keep it low and supple. You'll wanna dump the chassis as low as a 1/8 buggy (they sit low and hit the ground on every bump and jump). W/ the smaller wheels you can make the chassis and arms "level" (should look authentic). Dimensionally it sounds like the wide maxx arms will give you proper width, but perhaps the "narrow" maxx arms would make it "look" a little more authentic. Buggy's have a wide pan and short control arms.
And if you want to run it as a monster truck I'd get the OFNA 17mm 3 splits and Panther Komodo's. You won't have to change the hubs, just leave the OFNA 17's on. Put on a heavier spring (more clearence, like a MT), and throw your truck body back on. Viola! Back to a monster truck.