Is there a six speed manual trans that will fit an 1990 240sx?

Yes, a six-speed manual transmission that can fit a 1990 Nissan 240SX is the Nissan 350Z's transmission, specifically the Z33 model. However, to install it, you'll need an appropriate adapter kit, custom driveshaft, and modifications to the transmission tunnel and mounts. It's recommended to consult with a specialist or performance shop for proper installation and compatibility with your engine setup.