Antec provides all that is need to mount your parts into the Three Hundred Two.
I was able to get a clean looking install even with my non-modular power supply. I did run into a couple of things while doing my install.
With the Three Hundred Two being a mid-tower and having 8 expansion slots the motherboard is close to the top of the case. Close enough that my Prolimatech CPU Super Mega cooler gets rather close to the fan. So if your CPU cooler hangs over the top edge of your motherboard it is not going to work in this case.
If your motherboard is like mine and has the 8 pin CPU power connector in the upper corner of the motherboard then you will encounter the same thing I did trying to get the power cable plugged in. There is a small gap at the top of the case that I was able to wiggle the 2x 4 pin connector my power supply has through. If your power supply uses a solid 8 pin connector you may not get it routed this way.
Behind the motherboard tray I was able to run my wires and had no issues getting the side panel back on. Since my power supply is not modular I stashed the extra wires in the bottom of the hard drive cage. Also, since my motherboard does not have an internal USB3 header I just stashed the cable behind the motherboard tray for now. It would have been nice if there was a USB3 to USB2 adaptor cable, but if you’re building new there stands a good chance your board will have a USB3 header.