Best way I can think of to make the rack work with the straight six is to remove your oil pan and customize it for better clearance. It is a common hotrod trick. As long as you clear your crank, there is no problem at all by doing this. All you need is some mild steel sheetmetal of an appropriate gauge, an angle grinder with a cut off wheel, and a welder.
So you're not screwed. It's just gonna take a bit more work.
It just takes a little extra imagination, that's all.
