I'm not sure, Flipper... Very good question. I would guess that it's a compromise for the stock floor and mounting hardware. If you look under a 3rd generation Camaro, the long arm runs beside the driveshaft, not under it. I'm not sure it would make a huge difference, though, unless you moved the front pivot point up and back a lot which would affect weight transfer under acceleration/deceleration.

When you move the instant center up and back, it improves traction for launch/acceleration (suspension loads), but, if ya go too far, can cause wheel-hop on deceleration (suspension unloads). I think I would follow the Camaro's example and mount the center link beside the front u-joint.

Note also, on a 3rd gen Camaro at stock height, the short arms run slightly downhill from the rear axle to the subframe mounting points. That gives the stock Camaro a slight negative input from the rear-steer. I've never talked to any GM designers, but I would guess they designed it that way because the 3rd gen Camaros are slightly nose heavy and need the negatve input to control oversteer (loose condition). For your application, I think I would keep the short arms as level as possible and lengthen them to about 2/3rd the length of the long arm. That way the rear-steer input would be minimal and controllable. My reasoning here is that your car is a lot lighter than a stock Camaro and your weight distribution will be about 50-50; you therefore don't need a lot of rear-steer input under normal driving conditions. If you can somehow incorporate it into your design, it would be nice to have multiple mounting holes so that you could adjust the angle of the short arms. That way, if you ever want to run it through a Gymkhana or flog in on a dirt track at one of the vintage meets, you could dial in the rear suspension to your liking.