Does anyone know what driveshaft I need for a 904 to 8.75 rear

A driveshaft shop can shorten it easily. They aren't near you but I use these guys. Super old school.

https://carolinadriveline.com/
At least go to their site and use their forms to make proper measurements. Their forms are very good.

BTW - I am doing the exact same thing to my 73 duster next month. So if you wait I can share my results.

RGAZ