Can anyone please help me I live in London England and have a 1986 red C4 Vette

I have recently put a new crate engine in 325bhp and the engine runs fantastic my problem is that ever since the engine change my car has been reluctant to go into 3rd and overdrive on the auto box. Today i took it out and all I have is first and second gear and it will not change up the box any further than that.

Does anyone have any ideas what this problem could be. Since the engine change I have a new air pump, new water pump, new rad, new hoses, new idle air control valvle, uprate new fuel pump.

This problem is driving me nuts