Be careful welding the centre bearing mounts onto the body. The floor skin isn't the thickest and there's a good chance you'll have the carpet underlay catch fire.
My first car was a sedan converted from FWD to AWD (Legacy gearbox) with a custom 1 piece tailshaft, but then fitted a full-time 4wd box and needed to get those brackets welded on.
Easiest way was to bolt the brackets to the tailshaft, then fit up the tailshaft to the rear diff & gearbox, push the tailshaft up into the tunnel so the brackets sit flush, then tack weld them up. Unbolt the tailshaft & weld up properly (they're normally only spot welded on anyway).
Jealous of of these places that have the coupe available. I think there's only 2 or 3 that got privately imported later into Aus