I'm running a 502 block with AFR heads and Merlin intake and have the same problem. I have resealed it twice with permatex utlra seal. I sanded the block and intake then cleaned with alcohol, then put a nice bead of sealer on then the set intake on and lightly installed bolts then next morning torqed bolts to 30lbs and it still leaks. I have about .010 clearance between intake and block now I'm thinking of milling some off the intake to fit a gasket or more silicone.

That's pretty close. Did you measure before or after tightening the manifold bolts?

Do these engine's in question not have enough crankcase ventilation ?

Be sure and use an aluminium safe silicone such as ultra black as regular silicone sweats an acid as it dries.

If you are talking water leak, use blue Hylomar on both sides of the intake gaskets. This stuff was developed by Rolls Royce for jet engines and never sets and resists corrosion from forming. I have used it on all water surface gaskets for a few years and it never leaks. It is easy to clean ff also. Started using it on all outdrive mating surfaces and they are staying corrosion free also.

It also works well for blower to intake gaskets.

http://www.hylomar-usa.com/

Permatex sells it here at most autoparts in a tube

I just found it in a spray can which makes it really easy to use.

I measured after tightening the manifold bolts.
I'm running AFR intake gaskets.

SB.... I left the dip stick out to see if oil would blow out and no oil came out.

Got basic same motor and same problem. I do not let slicone skin over before putting intake on and still had leak. One weekend at the lake I tried a temporary fix by just taking a small tube of silicone and pushing in the area where it was leaking. Stopped the leak and have yet taken the manifold off and redone gasket from over 2 years ago.