whatever you bolt for a rear sway bar, whichever way you do it, without adjustability, its going to be very suck it and see anyway i suspect.
i was giving some thought to your purchased design bar, compared to some of the others that have been photographed earlier in the thread, and came up with the following theory!
this is where you may turn off and look for a more interesting thread! ha ha!
the sway bar you had purchased appears to have very little actual horizontal length in it, and in stark comparison to the red bar in the right hand pictures above, that has length across the whole axle, it got me thinking along the following lines.
for bars of equal thickness and identical material, sprung steel, the horizontal length is what provides the resistance, because its is this that section has to fight the twisting forces applied to it by the forward or rearward facing bars, acting as the levers.
for this reason i don't think it matters so much which way you mount it, either the horizontal section on the floorpan, or the axle, the forces upon it are applied the same whichever way round it is, and only the added or reduced resistance can change with your method of fixing, i.e rubber, powerflex, polybush etc tougher or softer mounts.
so is a longer horizontal section going to be stiffer than a short one due to extra material to twist, or would the twist resistance actually increase the shorter it is? i'm not sure actually, but one thing i'm convinced of is that the levers, the for/rearward facing arms at 90degree to the horizontal axle line, will make the biggest difference if you can make these parts adjustable mounts somewhow, you'll be able to play around much easier.
bored myself now