Richard, I found an HFS6v1 for cheap a few months ago, and considering using it as the HFS-4 is setup for DI.
I am also trying to keep the trunk empty as the only place for the Howerton would be in the back corner, and having it there would make carrying rifles a hassle. I am still considering putting a cheap PPS system with the pump mounted to the engine mount via a bracket. But I just found another HFS-6 for ridicucheap plus shipping, so I don't know yet.