No MT minitower is what you have and its psu is standard ATX form factor.  it looks wider than DT, but standard terminology is MT. the narrower form factor is DT, next smaller one is SFF. OptiPlex 960 Technical Guidebook Version 3 (dell.com)  
I'm planning on getting a new GPU for my wide tower optiplex 960, but I need a 365-400W PSU that's compatible with it. I'm only seeing posts for the mini tower, which might mean that it's a smaller mounting. I'd really appreciate information for a compatible PSU for this tower, thanks
Optiplex 960 PSU large form factor