It's after because you want to maintain the fuel pressure in the circuit the injectors are on. The injectors are essentially a controlled leak. This makes the fuel pressure more consistent and you won't have the restriction of the regulator before the fuel system. This is a bypass regulator setup, you can even run one this way on a carb and it's probably the best system for that also.
I did and it won't cover up the inherent issues with carbs like carb icing, bad fuel atomization on a cold engine, the inherent change of fuel levels in the carbs while coming up and down hills or going around turns, the effect of ethanol, etc. I like to drive my car, it gets cold here in the midwest before it snows and the carbs never liked cold running in vehicles without a crossover, especially when you factor in the stick shift and the tall gears. It's literally been two years since I messed with my tune at all.
I literally spent entire days working with accelerator pump cams, air bleeds, jets, etc on the QFT 750 Annular and the AFR was dialed in to run extremely strong, the issue is that when it's cold would either lean pop or the choke would have to be on, it just drank fuel in high vacuum, low load situations, and there's a drift in how it would perform due to other ambient conditions. I had the 4-speed in there but I literally went from 11 to 13.5 mpg overnight with the EFI and it drove better.
The amount of carb cars that run like absolute trash is astounding. My friend has a ratty 71 Demon that has a carb I tuned on it on it's 318 magnum and we actually had people commenting about how good the car started, ran, idled, everything at a cruise in, and even it's trash compared to EFI. It also has a properly set up distributor which is also another issue that people run into.
