I have used a propane torch to find vacuum leaks. You DO NOT light the torch! I would not use it around an extremely hot exhaust manifold. You just open the valve a bit and move it around until you hear the engine rev up, there's your leak. Make sure the air cleaner is in place as this will allow you to hear the increase in rpms better, and possibly hear the vacuum leak. The nice thing about propane is that it is heavier that air so it doesn't drift around and end up in the top of the carb, giving you a false reading. It works best if you turn the idle as low as possible and still have it run smoothly. That way you can hear the increase in rpms more easily. To get an idea of what to listen for when the propane hits the leak, aim the torch at the air intake of the carb and listen for the rpms to increase. You have already tried the "swap and hope method of auto repair" with the carbs without success. I would recommend that you find the real problem by checking all of the suggestions provided to you so far by members of this site before you swap and pray for the manifold to be the problem. There is no reason that a properly installed 216 intake won't work on your 235. I feel your pain, diagnostics can be frustrating. It is a matter of conclusively eliminating each possible cause at a time until you find it. Keep in mind that it may be more than one malfunction causing your trouble. Take a break, eat some red meat (vegiguy?) to get your mind right , and sort it out.
