Helicoil makes a head bolt specific insert which has a length three times diameter, which is the proper rule-of-thumb engagement depth for aluminum. Even so, it's impossible to bore a root diameter hole accurately with a hand drill, so about the best class of fit you can expect to get is 1B. A precision hole (one that would see high torques) is going to be 3B. You lose close to a third of your holding strength in that jump. Best case, a Helicoil is a "maybe it works..." proposition. It's a good job saver for things like spark plug holes, but it's not a high-precision, high-strength product. You'd have a much better chance at success with the Timesert, but I've seen those fail as well. As always, it may have had more to do with the person doing the repair than the product.
On the ARP studs, when I did my first Rover, I, as many others have, questioned the torque spec at 90 ft/lbs. I did the math and came up with 72 Ft/lbs, based on the 7/16-14 thread and 180Kpsi material used in their studs. I called ARP and spoke to one of their engineers, who couldn't explain the 90Ft/Lb spec. We traded some emails back and forth on the issue. He did mention that he felt that their 8740 studs in this application were closer to 185K, so I set the torque at 75 Ft/lbs and called it a day. In December, ARP replaced their spec sheet for the Rover kit with an amended torque spec at 70.
Bolt torque isn't some arbitrary number. A precision, torqued fastener is essentially a spring. It needs to be torqued to a specified point in the elasticity range of the material to ensure stable, lasting clamping force through all the operational range of the fastener. If you're not going to torque them to the proper spec, you can save a bunch of money and buy a couple sticks of all-thread and make your own. If the block won't hold the torque, it's a defective block and destined to fail, either by pulled thread or failed gasket.
I discovered that from the factory, the Rover head bolt holes are cut with a plug tap. That means that, at the bottom, the threads are cut at a decreasing rate (kind of like a pipe thread). The holes are a little deeper than the TTY bolt is long, so it doesn't matter. But the stud should be able to bottom in the hole and not bind the thread. If it binds, it can cause an expanding force in an already over-stressed area, especially on the deck's end holes. To alleviate the problem, you should cut that last little bit with a bottoming tap, preferably a class 3B, to get a good thread.