Here's a few tips to get the most out of your flash:
I would use that 35mm prime lens exclusively at the reception. The SB-400 doesn't have a lot of power, especially at distance. Using a fixed focal length will give you much more consistent results. It's going to produce sharper images as opposed to the kit lens as well.
The diffuser and bouncing serve the same purpose: to soften the harsh light of a small flash unit. The smaller the light source, the more harsh the light. The SB-400 is going to produce some nasty hot spots and shadow cast, almost as much as the popup flash.
How you choose to soften your light will greatly depend on the location itself. Ideally, you will want to bounce light off the ceiling. The SB-400 does not swivel horizontally, so you will not be able to take portrait-oriented shots or else your flash will go shooting off into Grandpa Joe's face standing next to you. Typically, reception halls have tiled white ceilings that are somewhat low. If that's the case, you're golden.
Keep in mind that the SB-400 is an i-TTL flash. It will not know you are bouncing, and your shots will probably be a bit dark. You will have to increase your exposure balance to compensate for this since there is no manual option on the SB-400 (to my knowledge). I would remove the diffuser if bouncing, just for the sake of getting the most amount of light possible. If the light seems too harsh, put it back on.