I have looked at some 3x2 TBI and DPI EFI systems, for my 440 6bbl and my wife's 421 Tripower GTO.
I had just sent my 3 good (not perfect) running Holley 2bbl carbs to Promax for ALL of their mods: jetable metering plates in the outboard carbs, angled idle mixture screws on the throttle plate section on the rear carb, their metering block on the center carb, repair 1 of the 4 screw holes on the accelerator pump diaphragm on the center carb. Also to rebuild any or all 3 of them with the correct parts if needed. I had rebuilt the center carb with a kit I bought at O'Reilly's, and ran it for some time, but I was concerned that there may be inherent deficiencies with that. I also had them "pretune" the carbs by running them on their dyno mule, a 440 with the same cam I believe I have...Lower and upper stainless steel braided fuel lines too....
So after making that $1,400 investment in 3 carbs that were installed and running before I sent them to Promax, then I discovered the FiTech 3x2 EFI system.
When I got the carbs back from Promax, it was a couple of days before Cruisin the Coast, an annual 8 day event that is a LOT of cruising around and the biggest event of my year. I had also just got the 3 Rochester 2bbl carbs back from being "restored" by a company in Florida, All American Carburetors. The problem with what they did was failing to modify the Rochesters to get the most out of the high performance parts, roller cam, etc that went into a recent rebuild of the 421. With all of the well planned, high end components that went into the 421 engine build, it should have performed better at the track, and I was still having some heat soak related issues too.
The Promax modded carbs RAN LIKE CRAP for the whole week of Cruisin the Coast, so I was pretty pissed off. It turns out that some braid clippings were in the brand new fuel lines I had installed but hadn't flushed out, because I was pressed for time and they were brand new after all.
Once I cleaned them out, and cleaned out the 3 Holley carbs, metering block, etc they ran FINE. The Rochesters still have to be sent to someone with experience and a great reputation for doing what needs to be done to make them work their best with a performance modified engine. If his modifications fail to provide the results we are looking for, THEN EFI in a 3x2 system is the next move for the 421.
That's a LONG way of saying that EFI can solve a lot of problems, with a lot less pain and guesswork.