I believe you have a vacuum leak somewhere and I'll explain. Could be a worn out carburetor. I've seen that happen many, many times and it usually means the throttle shaft needs new bushings...more about that in a moment. If at 45mph your engine stalls or dies when you put it in neutral and take your foot off the gas it tells me your engine is suddenly running too lean at that point. What you've done: taking foot off gas closes the throttle valve and this means the available fuel just went from richer to idle-speed lean pretty rapidly. I imagine the same thing would occur in your driveway if you were running the engine at say...2000 rpm and suddenly closed the throttle. Something just happened which caused your engine to get more air than fuel...too much more and enough air that your spark plugs can't fire the mix well. When your engine goes from higher rpm to idle, you get a vacuum spike of ~50% (say from 18 to 27 or 30)...if the engine is not in gear, this spike will be more temporary for certain, but it will be present just the same. If you have a vacuum leak, your engine (which is really just an air pump) will try hard to suck air from anywhere at all when you cause that vacuum spike. If a vacuum leak is present, that will be your culprit. When you have the throttle open, you're providing air and fuel. Even with a vacuum leak, you may still be running somewhat lean, but you probably won't notice a problem during normal operation. Vacuum leaks can be like that. When that vacuum spike occurs and you're denying fuel to the engine you force a very lean condition to happen. If your throttle shaft is loose, air can and will enter through the gap when your vacuum goes high. It won't have fuel to mix with (since your throttle is closed) and you'll get a stall. A vacuum leak could also come from power brakes and a leaky hose/bad power unit, vacuum windshield wipers and a leaky hose, a warped carburetor base where it attaches to the manifold, if your vacuum advance works on manifold vacuum that could be a possibility, leaky manifold gaskets can be a problem on SBC engines...etc, etc. Lots of things to consider. Good luck.
