Yes, you can use two right hand springs or two left hand springs or any combination you want. The spring rate between the two sides should not be different despite the difference in control ratio (leaf orientation) and camber(arch). I have never noticed any differences in left/right camber in XHD sets in any of them I've installed previously but it has been a while, I have to admit. SS springs, absolutely have seen radical left to right differences. XHD, not so much. FWIW, the left side is supporting a driver so they may have a slight difference in arch while the right side has to support torque, hence the leaf arrangement. FWIW, the E body set is splayed with the front eyes being narrower, whereas a B body set is parallel. No doubt this impacts both their leaf layout as well as their roll steer characteristics which is why they are not a duplicate of the B body design.

It used to be when you bought XHD sets, they came as a set. Only SS springs where sold separate. Looking at Mancini Racing, its appears you can now buy XHD springs separately. If you prefer to run two rights for a mirror image set up, then do it. Any shortcoming you may get from not using the matched set may not even be noticeable on a dedicated street car. IMO, 2" isn't much when you have a 3800# car plus potentially a couple hundred pounds of a driver and a couple hundred pounds of fuel. Plus, as mentioned, you can offset any imbalance in rear height with the opposing t-bar adjustment.

If you are happy with the feel and height of the stock 340 springs at 120# but need more control in the front segment, try adding a partial leaf on TOP of the main leaf from as far under the spring eye as you can get it to right up to the mounting saddle. This will both give additional support from upward, acceleration loading as well as downward, decceleration loading.