This is why it can take so long to get a multi-carb. set up working good, a bunch of small, unforseen problems like this can come up that need solving. Usually the interferance problem is between the underside of the intake and the rear header, right above where the header joins into the collector. It's hard to see, and if the interferance is slight, you may never see it. The intake could be held slightly off the head, causing a vacuum leak. To check for clearance, take a .020 or .030 feeler gage (or paper as on the road again mentioned)and make sure you can pass it between the intake and the headers anywhere you can see that they are close. If the interferance is minor, a little grinding of the intake or exhaust should help them clear each other. But never grind too much as you can go right through the manifold and turn it into scrap metal! It's been done before. Frank's idea of spacing the intake out away from the head will also work fine, but you may have to deal with uneven clamp flange thickness. Maybe this would actually work to your advantage with the reproduction Fenton headers with the thicker clamp flange?
On my original Fenton/Fenton combination, I had to grind on both the intake manifold and exhaust headers where they touched. Still not enough room to be sure the intake was tight to the head, so I angle milled the head side of the exhaust headers. By angle milling, I mean that I removed a small amount from the head flange surfaces, but not flat to the old surface. I removed more towards the bottom of the flange that bolts to the head. By changing this plane maybe only 1/32", the header tipped in towards the block an 1/8" or so at the bottom. I could do this myself and due to the time it would take to jig both headers up at a slight angle, a shop might charge $100 for this. A cheaper solution may be the way engine rebuilding shops resurface exhaust manifolds on a belt sander type machine. They would have to be careful to push harder on one side of the flange than the other to change the plane of the manifold surface that bolts to the head. This is very similar to what Joe is talking about to get the headers closer to the block and to thin out the clamp flanges at the same time.
No matter what or how you do it, care must be taken when fitting up an aftermarket intake manifold and Fenton headers on a 216/235/261. Don't rush through the job, take your time and see that everything is fitting up right and sealing tight. Too many people quickly slap this stuff on with poor results and then whine about how terrible it was and end up going back to a single carb. Fit the intake and exhaust, use identical carbs tuned for multi carb use, (see
www.inliners.org for more good carb. info.) and take your time in making up some good linkage that opens both carbs smooth and evenly. Exhaust or hot water heat to the intake helps (something I need to do myself!), and hook up a choke on 1-carb. for cold weather starts.
