Have removed 4EAT gearbox from the car. Torque converter and flex plate stayed with engine,is there a plastic plug on the right hand side of engine bell housing to access to flex plate bolts?

Yes or from the bottom.  Use a 6 point socket!

Make sure you read on seating the TC and you should inspect and or change the TC seal on the trans.

I personaly like to get um from the top. Go ahead and take the time and remove whatever you need to give you room. Be very carefull during install, don't drop um.