my 76 450 ran poorly (and stalled) when I bought it.
One issue I certainly had was a clogged fuel system - I ran a couple of bottles of Techron through the system and replaced the fuel accumulator.

I'm not sure how a vacuum leak would lead to the problem you had, except these cars are so prone to vacuum problems and that could impact the idle.

But try the Techron and take a look at your accumulator.