351C 2V intake seal leak. but why???

[travisgm]

Looking for a bit of insight as to why I am having issues getting this intake to seal to the block.
First a bit of history as to how I got to where I am.
My inlaw had he 73 restored and figured he'd finally put a 4 barrel and aluminum intake, chrome dress up kit on at the same time.
He had the parts installed on the motor by the person doing the restoration (I believe it was done by a different garage than the body shop).
Anyway, the person ordered a 4 barrel intake for a 351C, naturally they ordered the wrong one and got the 4V part instead of the 2V. He drove the car for the remainder of the summer, then ordered the right intake to be put on over the winter by me. I have put them on in the past on a couple Chevys and one windsor and never had a problem.
This Cleveland is confusing me.
As per the edelbrock instructions, I removed the valley pan (The 4V aluminum intake was installed with the valley pan left in) and used a nice layer of Ultra blue along the front and back of the block (I've never had an issue with blue before this). The intake gaskets I used were the "free set of edelbrock gaskets" that came with the new intake as a bonus.
After some spiritted driving, I noticed an oil leak coming from behind the intake pooling around the oil sending unit and eventually down the back of the block.
After doing some reading I here how terrible the blue is and I should use black or "the right stuff". I chose the right stuff. After a super cleaning job and making sure no oil is remotely near the sealing area, I put a nice bead on and the intake squishes it out evenly across the entire width. Letting it sit over night I go for a nice drive and see no oil, Hooray, it's fixed!!!!
Then I decide to test the new intake and carb a bit, when I return the leak is back.
Why is this happening, should I put the valley pan back in?
Why is there such pressure to be able to push "the right stuff out" after I read it was like super glue when installed?
There is a breather in the "add oil" driver side valve cover and a functioning PCV valve hosed in to the port on the base of the carb on the passenger valve cover.

Is it possible that this issue has been there a while but because the car was never driven "hard" before the carb/intake swap it was just never noticed until it has been hitting the higher RPM's?

My Inlaw is 73 years old and I'd say that he has been a "mature" driver with his car for the previous 10+years.

Bad rings be a possibility? I believe the engine hasn't been opened up since it left the assembly line. I am going to pull the oil sending unit and put a plug in for a quick test to make sure it's not coming from there (if it's possible).

What's my next step???

[Oztrailer]

I used the black sealant and don't have a turkey pan in mine but I don't have any leak so far.

[JTS71 Mach1]

It very seriously could be the oil sending unit, as they are notorious for leaking. I once did a rear mian seal on a car for a friend. After finishing it up, I let it run for an hour with no leaks. Then drove it to his place 35 miles away. About a week later he called me pissed and said the rear main was leaking. He'd had another mechanic look at it and said it was definetly the rear main. I told him I'd take a look at it. $8.00 for the sender and $10.00 for the socket. (I made him pay for the socket and I kept it) All was good as new. He even apologized for getting upset. So definetly check the sender! JTS 71 Mach1

[Steven Harris]

Good call JTS,

Could well be the sending unit.

[Oztrailer]

It very seriously could be the oil sending unit, as they are notorious for leaking. I once did a rear mian seal on a car for a friend. After finishing it up, I let it run for an hour with no leaks. Then drove it to his place 35 miles away. About a week later he called me pissed and said the rear main was leaking. He'd had another mechanic look at it and said it was definetly the rear main. I told him I'd take a look at it. $8.00 for the sender and $10.00 for the socket. (I made him pay for the socket and I kept it) All was good as new. He even apologized for getting upset. So definetly check the sender! JTS 71 Mach1

My oil sender was leaking when I first got mine.