What about fuel pump? Also check the hei ignition moduale. They have problems and fail all the time if its stock i would replace it. Coil is also a likely suspect as they have a tendencey to crap out every so often. Bad wires and aweak coil rpm fails and spark voltages drops it will drop out.
Does it feel like its pulling or tugging in gear. Also if you flat floor it and ask for max power from a dead stop does it respond as it should. Usally tq converter will not be happy under full throttle launch if the converter is shot.
If the trans is working fine other wise i would not look at it first even if it feels like the trans is holding the engine back. Many times a bad spark plug wire will make and engine seem like the trans is choking it out.