While changing the orientation of the carbs may make slight performance changes, the primary concern (at least in OEM setups) has been a matter of clearance and ease of linkage hookup. The only factory setups that come to mind with the fuel bowls to the rear are the FE Fords and small block Fords. In those cases it is physically impossible to clear the distributor with a forward facing Holley without relocating the carbs to the rear away from the intake ports. While the rear facing carbs cleared the distributor, it also resulted in a rather complicated and awkward looking linkage setup. The other popular applications using Holley carbs that come to mind are the Big and Small block Six-Pac and 427 Chevy setups which in both cases have no problem with clearance and had the fuel bowls oriented forward.