Martin,
The djet ECU uses the incoming air temperature along with the manifold pressure to calculate density of air. That along with engine rpm is used to calculate the amount of fuel to inject into the cylinders to achieve the required AFR.

Didn't understand the part about your fuel pump, but you do need a proper D-Jet pump, not a K-jet or an aftermarket type. And, it should put out at least 1L/30sec at discharge of Fuel Pressure Regulator. This ensures circulation of fuel back to tank at idle so vapour locking is less likely.

Interesting the 300SEL 4.5 doesn't have the vacuum switch over valve. Must have been something they introduced in 72. In your case, you would use A/C most of time, so why not disable the vacuum retard by disconnecting the tubing at distributor and plugging the tube? That's what the later M117s achieve by closing the valve.

I wish you luck.

__________________
Graham
85 300D,72 350SL, 98 E320, Outback 2.5