Dashpots should still be available, possibly through professional-level suppliers such as NAPA, not from the mouth-breather computer jockeys at Auto Zone, etc. A good-condition 4GC is a far better carburetor than either Holley or Edelbrock, etc. aftermarket carbs. However, I don't recall any 4GC's that were calibrated for a 350 engine. If you are determined to make a change, find a year-correct intake manifold and a proper part number QuadraJet. Our resident carburetor guru "Carbking" might be able to help you procure a dashpot that will cure your stalling problems. He runs a very professional rebuild shop, but Carter carbs are his primary specialty.
Edit: This one is probably what you need, but there were lots of slight variations:
http://cgi.ebay.com/ebaymotors/D5-2...arts_Accessories&hash=item3a3f9360dbJerry