What would cause oil blowback out of the dipstick and PVC on a Chevy truck with a 350 engine?

Sounds like something in the ventilation system is plugged causing negative pressure.

Engines that are worn, have not had oil changes on schedule or have been running "lean" can have some serious problems. If rings wear or if the ring groove is fouled with tar (burned oil) the rings may not be able to provide an adequate seal against the cylinder walls. If the exhaust gas can leak past the piston rings there will be a buildup of exhaust gas pressure inside the crankcase. The condition is called "blowby". The PCV valve assembly cannot possibly keep up with the exhaust gas that leaks past the piston if you have blowby. Unfortunately, severe blowby can push against the crankshaft seals causing the engine to leak oil.