I have a 65 GT motor with 302 guts, 4 webbers, uni-lite dist and MDS ignition.  It been running great for about 10 years and 9-10k miles.  Now it runs for about 20 minutes then shuts off.  I re-adjusted and balanced the webers and still no change.  I'm new to this site and mostly collect old Pontiacs...I would appreciate any ideas you have.  Thanks!  

Well the first thing to determine crazie is why did it shut off. Fuel is the first thing to look for. Whatever you have feeding those four carbs could be the culprit. I hope it isnt a stock fuel pump because it probably isnt up to the task. Look down the bore and open the throttle and see if you get some fuel flow. If its squirting fuel then look at your spark. Take out a  plug ground it on the engine and turn it over to see if you have a spark. Isolate it to one of these problems and you can go from there. In 20 minutes a motor just warms up, it could be the fuel pump, it could be your MSD ignition, or something else but you have to isolate it first by eliminating one thing after another.  

My experience with engines (especially Ford factory cars)just dying has been electrically related. (typically the dist. pick-up coil going open with heat). If it starts right back up, its unlikely the issue I related, but still may be several things like mechanical guy suggests.  

I agree with the other comments to determine if it is fuel or elctrical. Recently I had a similar situation with a newer vehicle (1986) that would suddeenly stop running and then after a couple minutes or sometimes longer would start. I checked fuel, spark, replaced parts and then some one suggested the Ignition Control Module...don't know if this is something that may have been added to your vehicle, since it wasn't OE on '65 vehicles. I was told that intermittent ops is the first sign of failure. They are relatively inexpensive. Since I replaced mine, no problems, Neil