I would go the other way, ie. 5x8.
The shorter seam will cause less interuption of the material, less loss, and assuming it is seam sealed, can go either way while setting it up, assuming you got 60" fabric.
Cat cuts work OK for single purpose shelters. The nice thing is they set up nice and tight without a lot of flapping that can loosten stakes.
I prefer a flat tarp because of the multiple options they provide. ADK lean-too, wheelan tarp with wings, diamand and flying diamond are a few of the most common I use. With the wife along or another partner, I set up as an "pup" tent over a mesh body for bug protection, thus paying a little in weight for versatility.