My money would be on the china walls. When I installed the 4-barrel on my 318, I didn't use a tall enough bead of rtv on the china walls and after several heat and cool cycles I was leaking oil out the back. After getting advice on here, I was able to "fix" the leak without having to pull the intake again and start over. I pulled the distributor and oil pressure line and cleaned the back of the intake and top of the block with brake clean thoroughly and then basically placed a "bead" of ultra-black along the intake to block joint and smoothed out with a rubber-gloved finger. No leaks since and I've probably driven a couple thousand miles. I found that some guys actually fill any remaining intake to block gap with rtv after installation and before the engine is ever run which I will do in the future.
