New 5.3 LM7 low on power

[70TWO NOVA]

Ok guys, looking for a little guidance. I rebuilt a 5.3 lm7 with all stock bottom end, new bearings throughout. New 4.8l pistons, ls6 cam, intake, tb and injectors. I fired the car for the first time the other day and drove it for the first time today. The problem is, it is extremely low on power and has virtually no throttle response. Engine rpm barely increases even at 3/4 throttle or more. Iam stumped here. Any ideas as to why it would seem to be so low on power? When the car is in park or neutral it will rev up fine though the RPM range and sounds to be fairly healthy.....

any insight or help is greatly appreciated.