A question was asked about the "power valve". The "power activating piston" and its function are described here: http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0659.htm
The piston can be cleaned and polished with some very fine sand paper(600-1000 grit) and a hand drill. chuck the long skinny part into the drill, polish the piston part until you have a smooth fit in the body of the carburetor. It must be smooth and free, all that moves it is vacuum and a spring. Usually only moves at full throttle under a load. But it can move under partial throttle and loading as vacuum demands. If it starts and runs most adjustments can be made. Even with a poor missing idle you should see a Dwell angle of about 30 degrees. That confirms the point gap is correct(0.0018 to 0.0022 inch). More "Specs" here: http://www.tpocr.com/chevr.html
The stuck heat riser must be fixed. So do that first. After that is working start with the suggestions as posted above. Lots of smart willing folks here to help you. Keep us posted.

Steve H