JT's advice is sound, get everything setup and all the weight (engine in the car) prior to fitting the flares. I have yet to see a decent set of steel GRP4 flares, they always look too flat IMO as it's hard to get the compound curves. Larry Stock at Pantera Parts Connection has access to a great composites guy here in Reno, NV, pretty sure he could do the flares in carbon.
On my cars I have adjustable rear upper a a-arms that counteract the negative camber. That said, if I were in your position and starting from scratch with aftermarket wheels I would lengthen the a-arms and offset of teh wheels accordingly (the later Si Pantera's had such that shifted the wheel center line out counteracting much of the tram-lining tendency of the GP4/GT5/5S cars).
There are a few folks in Australia and the UK that have completed a RHD conversion, it isn't that difficult to execute on a bare chassis, either move the pedal box over or duplicate on the right side if messing with the VIN stamped on top of it would create legal issues. If keeping the later one pod dash it can be cut and rejoined to form a RHD or I recall someone had a RHD mold fabricated at one point. Joe Nardo in Australia is a wealth of information and would be a good contact, he also has among his 8 or so Panteras an Si for any measurements!
Julian