The heat valve was very stuck on my 53 when I went to rebuild the engine. We ended up cutting the inside valve part out and leaving the shaft with weight and spring on the outside. The engine runs fine that way.
Before going into the exhaust, take a look at the tune up. The point gap, initial ignition advance, and particularly the vacuum advance are very important for a good running 215.
Also, be aware that the available head gaskets are thicker than original and some of the rebuilder pistons used over the years are lower compression than factory. I lost power when I rebuilt the 215 in my 53 sedan, but it runs much smoother and doesn't leak all over the place. Without modification the 215 will not quite have the power of a good running V8, but will drive you anywhere.