10 Day Magnum Swap - '65 Dart 270
The magnum gaskets are exactly the same as the 318/360 ports with the EXCEPTION of 340 HP. I have downloaded a photo of the 318 drivers side from my 67 GT Vert we just yarded for 5.9 magnum swap. We sell 4-5 pairs of EQ's a week and use them on almost all our cars and they are the same as magnum head with gasket match.
I stand corrected.
Felpro shows a 318 specific gasket for motors '74 and older (MS9939), and a different one for 340/360 motors of the same vintage (MS90460). After '74, all 318/360 motors use the same gasket as the 340/360's did in the prior years (MS90460), until the Magnum motors hit the scene. The magnum motors use a different gasket (MS95480), too.
Based on this, I can see why you would say the 318 and 360 share the same exhaust port, and I would probably agree provided you mean 360's produced after 1974. In '74 and earlier, there was a different gasket, and I would argue that the 340 and 360 are the ones that share an exhaust port. It's possible that Mopar didn't change the port at all after '74 on the 360's, but just used the same gasket on the 318/360 regardless of the port size or mismatch, but I would bet that the port did change and you are right that those year 318 and 360 share the same size exhaust port.
Regardless of the above, if you buy an exhaust gasket for a pre-Magnum smallblock and run it on a non-OEM Magnum head, you will have the issue I ran into. The gasket they sell for the '78 318 is the same one they sell for a '69 340, and is the same one I showed in the picture in my link. That gasket is too long and will not have a surface to seal against on a non-OEM head, regardless of what manifold is used.
But, what you are doing makes sense. If my above extrapolations are correct (a stretch, I know), all 318 manifold and '75 and later 360 manifolds will work well for the swap, regardless of what head is used, with the need to use a magnum gasket being the only real "gotcha".
On a side note, I think the Magnum has proven that the 340 exhaust port isn't necessary to make power. Which makes me think the only real advantage to the driver's side "freeflow" HP manifold is the size of the outlet (2 1/4" vs. 1 7/8" I think?). And I understand that the passenger side freeflow is equaled by a Magnum manifold. So, maybe I could dump the sheetmetal adapters I am running by moving over to a passenger side Jeep manifold (fits better than a Dakota), and opening up the outlet on a 318 driver's side manifold. Then I could sell the expensive HP manifolds and end up with a better package (no adapters) and some cash in my pocket. :D
