What about getting an early efi m110 setup... or in your case, just the head and parts attached to it, then get a custom computer, like a megasquirt (low cost, with all the features of the big buck computers)
www.msefi.com. This is what Im doing to my 1968 w108 280s. Also putting a supra 5 speed in while im at it... getting bigger injectors is fairly cheap, just ebay some nissan gtr injectors, i got a set of 6 for $AU100. this will eliminate some of the carby lag you get with drawthru setups...
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1968 W108 Mercedes Benz 250SE <- sold
1970 W108 Mercedes Benz 280S <- project underway
1974 W116 Mercedes Benz 450SE <- serviced and awesome!
1977 W116 Mercedes Benz 450SEL <- sold
1976 HX Holden Statesman Deville