I also just bought a 87 300sx I pulled the motor last night. I am a little worried that 87 300sx hull does not have the motor mount capability to run the twin cyl. I also have a 86 js300. I put a 440 in it this spring. It had the places for the motor mounts in the hull for the twin cyl. I have been looking All over the net for a pic of the 550sx motor plate. but all i see looks just like the motor plate for 550js. So take a close look at the hull. U might have the same prob I have.