[blykins]

An engine of that caliber shouldn't need head studs, bolts will provide enough clamping force.

Also keep in mind that if you switch to studs, it could cause a different area of cylinder wear if the block was honed with a torque plate and regular bolts (or without a torque plate at all.)

I would make sure that the head bolts have not backed off, then do a leakdown test as Vector said.

Intake gaskets can really screw things up too....especially if something like a Felpro Printoseal intake gasket was used.