[x-bird]

For locations, being on top of the bearing hangars is ideal, since that's where any rising force from the tires/axle will come in. putting the mounts in between the pivots and hangars moves that force to the swingarm bars and stresses them. But they are beefy enough to handle it if you need to go there. For the most part, this line of thought primarily comes into play on hits that cause the shock to bottom out. likewise up top, you should add some tubing to the rack tube that carries up into a solid junction. View the force as an arrow going up through the shock mounts, and think about where it ends up. for my builds i do beat the living heck out of my suspension so i make sure the impacts do not end up in unsupported tubing runs. from day one of its life with me --and even before, as a 3203, the rear shocks were cracking the frame tubes, likewise with the rear shocks on spiderboxes and just about all other buggies. the upper mount sitting on a straight tube run with a shock driving into it is a far from ideal design.

As you can tell i love to tinker with things, for me, rebuilding/tuning the shocks gave me a lot more knowledge and ability for the future. I still strongly recommend doing them yourself, they are not difficult to do and add to your arsenal so to speak.
I did have a long background with working on a variety of mountain bike suspension forks and rear swingarm shocks from nearly 30 years of racing them both xc and DH and also was deep into the automotive fields before i got into buggies.

As far as getting close to vertical, as long as the swingarm cannot kick the shock beyond the peak of the arc and make it unload/push with the result that the swingarm locks upwards, that's what you want. the more you "lay" the shock off vertical the more stress you put on the shock shaft, it increases stiction, impacts damping and can lead to shaft bending and wear on the side seeing more thrust.