You should use an adjuster each side of the gearbox to stop it twisting.
That would be my advice, but I did not do it. I wallowed out the lower transmission bolt holes in the plates for the BAR so that the box could move slightly. I then drilled another hole quite close by and installed a mushroom that was tapped for a 1/4-20 bolt.
The mushroom sticks through and is 1/8" proud. Hole "C" is the lower primary cover mounting hole. I cut a 1/8" strap and drilled a hole for the mushroom and one for the transmission bolt.
I made two more straps, one with the hole punched a little further away and one with the hole punched a little closer. Between the three, you should be able to adjust for any occurrence. The little holes are for wiring.
The mistake I made was having the head of the bolt on this side because you have to pull the bolt out to change the strap. I had a reduced shank for clearance on the other side so I could not swap. I used the so called "mushrooms" because you do not want to use a nut where the chain runs. The chain can damage the thread and nut and you will have to break the bolt to get it off. I thought it better for the bolts in the chain area to have these mushrooms for nuts with no thread poking through. They will always come undone.
I think Tim is correct that you can get away without a tensioner. I found that with my original set up the belt was too tight, so I needed a way to loosen it. This is quick and easy. Once the drive side was held steady, the other side had no issues holding steady, too.
I love the avatar, but all the lads will be emailing you with their requirements.
David