i just recently plugged/crimped my egr hose on both the manifold side and header side. by doing this, if the dipstick tube is not secured enough inside the block, will pressure build up enough from the egr blockage to cause oil to go up the dipstick tube/or through the hole in the block. the tube is somewhat loose, i can wiggle it a little. it seems i have some oil on the y-pipe and headers, also a little on the spark plug boot. also, the tube is not attached to the block...

 

The EGR does not interface in any way with the crankcase. It connects to the intake manifold, and all the exhaust gas flow eventually goes into the intake runner in the heads, not to the crankcase. I can't think of any reason blocking off the EGR riser tube would overpressure the crankcase.

Usually, crankcase pressure is a sign of a clogged PCV valve, or excessive blowby on the piston rings. In the LT1, even this is rare, since the air supply tube from the side of the throttle body to the passenger side valve cover will reverse flow and vent the excess crankcase pressure into the throttle body/intake maniold.