cammed or charged LS3.
if you really want my opinion
bore it and stroke it, chuck in a massive cam, and so on.
neighbour used to have a 5.7 charged. now has a 6L bored and stroked to 7L. both had full exhaust, massive cams, pretty much the works (motorsport spec)
he much prefers the 7L as it has so much more pull and way more grunt.
and see your car is "out there" this would suit it
or this may be good, although it is probably way way out of the budget
http://www.ebay.com.au/itm/260601242214?hlp=false
but i really like what youve done to this car. looks really nice.
cant wait to see what you do next though