There could be several reasons...to much pressure caused by the Jet (could) blow out the Head Gasket if things arnt flat. Did your shop Deck the Heads and Block for true flat? Did you assemble the motor...torque the Heads correctly?
The list could go on...but I would check for true flat Heads and Block...and check to see how much pressure is in that motor...this can be dome with a guage at the Intake Manifold.
With my Bassets and Rex Thermostat Bypass...and 2 dumps out the back VS most jets have one...I'm seeing less than 9 psi of water pressure.