I'm not sure why you want to use film other than ASA 100 or there abouts unless you are wishing to have a grainier look.
The other thing is bouncing the flash off the ceiling is still not a very flattering or great way to photograph someone. If you have a cord for the flash instead of a hot shoe (camera mount) you will get better pictures if you can mount the flash on a stand a few feet to the left or right of camera and slightly higher. Then as suggested, diffuse it a bit.
If you're unsure of the output, bracket the flash instead of the camera. (That is change the flash setting on identical shots they pick the best one in printing)
You also didn't mention if you are blending the flash with ambient light coming on from windows, in which case the flash would only need to be an accent light.
Also if you have time, get a subject and fake the whole thing beforehand to identify what you need to do. Print those shots then when you do the real shoot you can concentrate more on the subject and less on the equipment