Trunk mounted battery

Yes, you need a ground wire from engine to frame. It needs to be close to same size as your positive cable. If you have an inadequate engine ground, you can cook things like speedo cable, shift cables, etc. Power will search for a ground.

I run the battery cable inside the car, under the sill plates. Running a kill switch is a good idea.

I build 1/0 battery cable in varying lengths.

STAY AWAY from chinese wire. The wire is not good and insulation is marginal most of the time. I've seen wires labeled as one size with FAT insulation in it and the actual wire inside was 2 sizes smaller. Looked like 8ga outside diameter, wire inside was 12 ga. Buy wire that is manufactured in the US or Canada.