Slant six Holley 2280 stalls when stopping

Try tightening up the carburetor mounting fasteners to solve that leak. It's very likely not the throttle shaft. For those to leak, they have to be really BAD. Incoming air takes the path of least resistance. Coming in through the carburetor throat is a HEAP easier than coming in through the throttle shafts. Normally they will not leak unless they are floppin slam around.

That camshaft has 219* duration @ ,050" lift. Not Earth shattering, however, for a slant 6 that's a good bit larger than stock. That means it will want more initial timing. I would try pulling the initial timing up to 12* and see how it runs. If it's favorable and you get no spark knock (pinging) pull in a few more degrees like maybe to 15* and retest. Also, if you can get us a manifold vacuum reading both before and after you change the timing, that would be good. Lastly, did you degree the camshaft, or just line it up dot to dot? Lastly, put the vacuum gauge on it and adjust the air screw on the carburetor to achieve the highest reading. Do this in drive if it's an automatic. Parking braking on and wheels chocked of course. Try these things and report back.