I just did the conversion and clipped off my old harness and tied directly into the stock harness wires. When doing it with the 8239 the wires are as follows:
Subaru > MSD Harness
Yellow - Green
Black - Black
Red - Blue
And that keeps the plug on the same side but the MSD logo will be on the back side. The bolt size you need is M6 x 45mm or 50mm and you only need 2. Since mine is a DOHC I went with the wires for a 98 Legacy and you don't need to mod them.