As I understand it, the purpose of the different motor mounts for the big block was the provide some exhaust clearance around the steering box. The mounts shift the engine over and up slightly to improve the clearance for the exhaust manifolds & pipes. No doubt it helps the spacing with the power brake booster as well.
Similarly, the heater core outlets were shifted closer to the fender to reduce the tight fit and awkward hose routing.
To accomodate the engine shift, the transmission crossmember is also different and has the mounting holes off center. This is to keep the engine and tranny in line. Check out the link below for the last one that was on Ebay.
http://cgi.ebay.com/ebaymotors/ws/eB...44942&rd=1
As far as between the tranny and rear end, that isn't an issue since the U-joints deal with the slight mis-alignment.
Since the 67-69 Camaro and 68-74 Nova share a common subframe and firewall layout, the same logic applies to the big block Camaros.