It depends.
Externally, you can add an a/c compressor and front accessories, power steering, etc all the way up to 1984 when they axed the motor.
On the intake/exhaust manifold and carb side you can only go up to the mid 1970s when they switched to an integrated head. I suppose you COUlD swap the integrated head and 2bbl on there. But why?
With the traditional head you can buy a modified intake, either tri-power monojet, dual 4bbl, or single 4bbl. But beware, without mods they can be known to run hotter in different t cylinders than others because of fuel distribution down the intake, and, thus vale wear uneven and problems down the road.
If you wanted a header you need the non-intgrated style head.
As for not feeling anything for a breaker bar. If you have front accessories and pulleys on the engine, there is a bolt going into the nose of the crankshaft that you use to gently rock the rotating assembly back and forth to try freeing it up.