Hello. There are several things that can cause this. You need to start by checking to see if the shift linkage is moving on the transmission when you try putting it into gear. If not then the linkage may not...
Depending on the HEI system you have you may have a weak coil or the carburetor is running out of fuel at the higher rpms. Use a spark tester to see if the coil is capable of sustaining the spark...
