is it safe to carry a black tip v8 on a J bar roof rack

I'm going to look at a used V8 this coming weekend-- if it's in good shape I intend to buy it. Unfortunately I've got Yakima racks on my subaru hatchback with J bars, and since it's a hatchback there's not a lot of spread between the bars. The boat is about an hour and a half away from me, practically all freeway driving. I'm going to get a Goodboy roof rack extender, but there's no way I can have it by the weekend.

Suggestions? Is the performance construction up to this? I've been carting around a 16' carbon- Kevlar kayak with this rig for a couple of years, but that's a single- skin boat with no core to crush.

My only experience is with a V6 on my compact sedan, but I don't see how it would be a problem if you have good bow and stern lines. I found having a bow line going to each side of the hood really keeps the boat from moving, even in very high winds at highway speeds. The straps on the rack should never be made tight enough to compress the hull. I bought a bunch of cheap neoprene waist belts with velcro closures (sold as weight loss gadgets) that I wrap around the upright portion of the cradles for extra padding.

Lately, I've been loving the Thule K-Guard racks. I don't think they're officially sold in the US, but can be found on eBay and elsewhere. The boat sits upright in the cradles during transport, but the cradles rock back to put the boat on edge for loading and unloading.

I also think it should be fine. Like jsapan said, do the side to side ties on the front and one on the back.
Picking up a Goodboy eventually is a great idea, it's so much quicker and easier to use with a lightweight boat.