This would be a very long story if I started from the beginning so I will cut to the chase.
I bought a rebuilt 383 and ran it but it had a bad vacuum leak at the intake. I tried performance intake gaskets, swapped intakes, swapped heads, and had the deck checked. I am at a point where I am pretty sure the block was heavily decked to get the pistons close to zero and now the intake probably needs to be milled.
The question is, if you don't know how much the block was decked, how do you figure out how to mill the intake, or can you?

First thing is to check the intake fit to see if it will sit flat on both heads, you can do that by removing the tin intake valley pan and scrape and clean the heads and intake mating surfaces to make sure no gunk is on the mating surfaces. Set the intake on the heads and see if the intake bolt holes are aligned in the middle of or on top or bottom of the holes intake bolt in the heads. Once you see where it is sitting write it down, then take a set of feeler gauges and check the front and back sides of the intake manifold at the top and bottom of each corner, see how much clearances each one has, I have seen some have .000 on one corner and .037 on the one corner in front of it Write down the clearances on each corner, top and bottom and then take the intake manifold to a good machine shop and have them surface the manifold so it will sit square on your motor If the maifold is aligned on the bolt holes on both centers your a very lucky man If not getting it centered is a hit and miss process If the intake manifold holes are higher than the heads are (after squaring it up)then the intake needs some more cutting, if they are lowered than the head holes you'll need some intake thicker gaskests, put the valley pan back on and add one gasket per side and take a look Good luck, BTW did you try using WD40 to help find the location of the vacume leak? If not you can squirt it around the intake ports with the motor running and when the motor picks up RPM you are near the leak Let us know what you find, it will help others on here also

anytime you remove material from the block deck or head surface,you have to remove material from the intake side of the head.I think for every 0.010" removed from the head or block,0.012" should be removed from the intake side of the head

Just a mention, as I've seen it done in the past when individuals start swapping manifolds on B/RB's, it's valley pan first, then gaskets if needed, not gaskets, then valley pan...

Another thing to mention is that the top of the block needs to be milled as well when the decks are heavily milled. I've come across some shops that don't think you need to do that on a big block Chrysler. If you don't it will cause alignment issues and distort the valley pan which then will leak air.

I think we got it fixed. I had .014 taken off each side of the intake and reinstalled it and checked it with a smoke machine/vacuum leak checker thing. No more leaks there so I am getting ready to put it back in the car and run it.
We'll se what happens...