I just finish rebuilding my 86' 5.0, I stroke it to a 347, and install a cobra upper and lower intake, it seems to have a vacuum leak in the crank case, you can hear it and feel the vacuum from the oil breather on the valve cover. Any thought ???????

You possibly have an intake leak.  I've experienced situations where the intake gasket didn't seal on the under side of a port and caused it to pull a vacuum on the crankcase.  This could be caused by too big of an intake gasket port size, incorrect placement of the gasket, too much silicone around the ports, incorrect torque procedure, and a few others.  Not an uncommon problem.  Before taking the intake back off, though, pull out the pcv valve and see if it stops.  May simply be a bad pcv valve.

Thank you for the info, it was a intake gasket that it move down, I repleced it and put a better sealent to hold the gasket, it works great.