It could be leaking past the intake gasket section which covers the open casting holes in the head found on each side of the exhaust crossover, between each intake runner pair. When you pull the intake & gasket, you will see the pushrods. However, I have never experienced a leak here, but it could be possible. I would make sure that the mating surfaces on the intake are "true" and not warped or damaged in some way. Clean all mating surfaces thoroughly.
When re-installing the intake/gasket, make sure you torque the intake bolts evenly - BUT
, there is a long bolt at the front of the intake-to-water pump that applies pressure to the rubber O-ring that seals the water pump to the intake. Clean both surfaces, install the new rubber O-ring seal, and torque this bolt first
to draw the intake forward and up against the rubber O-ring to seal it. If not done this way, you may have a water leak. Once done, then install & torque down the manifold bolts.
Another place to check is your valve cover gasket. These can leak and drip down past the intake manifold and go out the back at the distributor. Often, if the original steel valve covers are used, people will over tighten the small bolts and crush the gasket and dimple the valve cover holes downward. So when you go to install a new set of gaskets, they don't clamp as well as they should and can let oil leak out. Always straighten these holes where the small bolts go through - make them flat and even again. Valve cover "hold down tabs" can spread the clamping force over a greater area to help prevent the valve cover hole crush. I have used the small type, but they come in a variety of styles if you want to dress up the engine: https://www.summitracing.com/search/...hold-down-tabs
stated, oil could be coming out of the valley pan under the intake manifold, but would be hard to tell without removing the intake. Also check the rubber grommet found in the valley pan which your PCV valve fits into. Over time these dry out/rot and oil could be coming from a bad grommet. You should be able to see this if this is the problem.
....and oil could be coming out from under the distributor housing base where it goes into the block, but if your mechanic said it is going past the distributor housing, then it would be some place forward of it.
DO NOT USE any form of stop leak or other additive - just not a good idea if you plan on keeping the car. Fixing it right is really not a big job.