the B18C1 is a Bseries motor. Your stock motor is a D15B7 (most likely) and is a Dseries motor. You can't swap a Dseries tranny onto a Bseries block and vice versa.

You will have to buy a tranny for a Bseries either way so you can either go manual and do the conversion (which is a bish) or you can just keep it auto.

Btw, a B18C1 is a GSR so that's like saying an EG 5th gen.. it's redundant.