You don't really describe the problem much other than to say it is a vacuum leak, but there are other things that can make an engine seem like it has a vacuum leak, so keep that in mind. For instance a bad accelerator pump valve can hesitation and misfire. An EGR valve that is stuck open (I know you said you are not running one, but just as an example) can have the same effects as a vacuum leak. Believe it or not even a mis-sized PCV valve can seem like a vacuum leak.
You should really get a vacuum gauge on it. They are pretty inexpensive and a very telling and handy tool on these older cars. As a general rule most engines run at between 16 and 22 inches of mercury (there should be a standard for your engine some where on the internet, or in a service manual). Anything lower than that is a vacuum leak, or problem like I mentioned above (like the EGR). If the vacuum gradually drops it is more than likely an exhaust restriction. Vacuum that jumps around usually means you have bad valve guides, or something wrong with a valve. I had a 79 Ranchero GT with the 400M engine, and it had the fluctuating vacuum, sure enough it was a cracked intake valve. There can even be wear on the throttle shaft on the carb that can cause noticeable leaks.
I would also check anything that you have done as of late that could have caused the issue. I wouldn't assume that it was a head issue, unless you had severely over heated lately or something like that. I would however like mentioned by someone else be suspect of the carb space. They are more often than not a problem. Hope this helps.