can i put a V8 904 bell housing on a V6 904 and bolt it up to a 318?

I do not know. The 318 has a particular balance on the torque converter I believe it depends on the year. I believe the earlier 318 torque converters were neutral balance and maybe the later years were weighted.

I dont even know if the /6 tc has a higher stall and if it would be worth it regarding strength - durability.

I just dont know.