When my car had the 355 4speed in it i broke a motor mount and the engine rocked enough to make a LOUD knocking sound but it seemed more from the middle of the car (header hitting floor). Maybe check the rearsprings like has been mentioned, maybe they are week and either hitting the bumpers on the traction bars or spring wrap-up on launch.

William

I would like to check out some cars in the houston area sometime, maybe in the spring. I'm at A&M and my car should hopefully be operational by spring time (LT1/T56 swap).

 

I had this exact problem in my 1982 Z28 after my engine swap, with acceleration, the exhaust traveling through the piping rapidly increases in speed, causing a shift in the exhaust, if the piping is not fully mounted in place it will hit the floorpan between the catylitic conv if equiped, and the rear axle, are this is where the pipes are closest to the floorboards. check you exhaust.