As I understand it, you'd want the sidecar on the right for a coupla reasons... As you're a bicycle/low speed vehicle you'd be on the far right of the road and with the added width of a sidecar you'll be ditch running from time to time & better to have the 'car in the rough than the driver lol. It'll also help with visibility - cars are more apt to see the taller driver sitting upright than the sidecar stickin' into the lane.
TBH I would be a touch concerned w/all the added & sustained strain on the bicycle components at the speeds you hope to achieve, there' also something about a sidecar's wheel placement... I think
a sidecar typically has it's wheel placed slightly forward of the bicycle/motorcycle's rear wheel for improved handling & stability, but I don't actually remember the specifics
There's also the fact that if you attempt to drive both rear wheels, posi would be the simplest... but as a sidecar bike already has stability issues while turning, that may get decidedly interesting heh