The 73 450 should be an EFI car. The EFI system is a speed density setup so it doesn't measure airflow. It measures intake vacuum, engine speed, and somewhat archaicly throttle position. It will not run properly on the larger engine without corresponding adjustmants to either injector size or fuel pressure. That being said the EFI on the 73's was the WORST injection system ever put on a car. The parts are prehistoric and terribly expensive. To be generous I will say that the 73 EFI when working at 100% of it's design is marginal at best. This was the first serious attempt to make an EFI car. To swap it onto a 560 would be like replacing your pentium processor with one from a commodore. If you are seriously considering modifying your 73 from stock the first thing to go should be your entire fuel injection system.
By the time you convert your SL to another fuel system along with another engine (and tranny) you will realize that you would have been much happier simply buying a decent used 560SL. Although they are the same body, the 560 is superior in almost every way to your 450.