yea stay away from mantapart.. jbody guys have known this for years when people stopped recieving their parts from them.
if you want to get serious about n/a shit, talk to ray bates at bates engineering or talk to karo at allecotec.com
M2 Race Systems - CNC Ported Cylinder Heads & Custom CNC Porting
do great CNC port jobs that flow really good numbers
makes a bolt on ITB kit for the ecotec. hows four 55mm throttles sound?
karo can get custom made high compression pistons and I know he does some other specialty cam stuff by copying european cam profiles... also, if you're looking into rocker and lifter upgrades goto JESEL Valvetrain Innovation
.. but be warned, upgrading to roller rockers and solid lifters allows you to run custom cams with larger lift numbers, but will cost about $180 PER VALVE
the big drawback with doing an all motor build is that majority of the time you need to have custom machine work done. its not a simple bolt-on affair.. you need a machine shop that knows what they're doing and is at least somewhat familiar with the ecotec.
a zero-deck will help raise compression up slightly too, but you lose a bit of valve to piston clearance, so you either need reliefs (especially true with domed pistons) or you need to limit yourself on valve lift.
the biggest thing with n/a is everything needs to be optimized to work TOGETHER or else you're just wasting time.