C6 just rebuilt, Transgo C6 spring kit, forward and reverse normal, won't shift 1-2 in drive or 2-3 will shift manually but no high gear. governor? valve body? it did have you drill the separator plate and drill through a partition in the valve body " lube upgrades"

 

Sounds to me like either you don't have vacuum to the modulator or the kick down is stuck. Also check your screen on the top of the valve body and filter if you pull the pan back off to see if you have anything in them.