oil out of back of engine onto hot pipes why from where

Sounds like either a rear main seal, external head gasket leak, or valve cover gaskets causing the oil on your exhaust pipes. The thunk on acceleration is probably U-joints since the Corvette is rear-wheel drive. Have the U-joints checked for play before suspecting the transmission.

If the oil is coming down the back of the engine, it is the valve covers leaking. The clunk could be in the drive shaft u-joints or the half shaft u-joints. Not a bad idea to replace all of them at one time if you do. Also check the distributor for the source of the oil leak.