Personally, I wouldn't run anything that is throttle body injection, but my performance expectations for the effort and cost to retrofit fuel injection may be higher than most. IMHO, sequential multi-port fuel injection, complete with computerized distributorless ignition and using all OEM Ford parts has been an extremely reliable and troublefree path for my 400.
From nearly every source I've read, Holley has amazing technical support, and will often cover stuff out of warranty, whereas FiTech has horrid technical support. I concur with the Holley support, as they've been phenomenal with any issue that's come up (which have all been very minor).
I have the Sniper on my 300 and love it. Yes, the 300 came with MPFI in 87 and I could have swapped all of the components on, but in my case the Sniper was the better way to go. Not sure how the 400 is, but the 300's fuel injection system can't support much of anything past stock, and there isn't any aftermarket for it. So, with the build that I have, a tuneable/programmable system was the way to go.
Installation was straightforward. Just do not take ANY shortcuts, and read ALL the instructions. 95% of issues are due to installer error. Learn how the system works, learn a bit about fuel injection, etc. so that it isn't some "magic box" sitting on top of your engine. Do a good, solid, complete job on the installation and you'll be happy.
In my opinion to do multi port injection on a 400 is a waste of money. You would have to have the manifold drilled and tapped for it, and that's just to start. Custom made fuel rails, all the eltronics. Nobody makes one and the 400 never came with factory refi. If you have a windsor motor or 460 they came with refi from the factory.
My 400 has a factory Ford Lightning 351W lower intake manifold which is setup for multiport injection. It is adapted to the Cleveland 335-series engine using Price Motorsports adapter made of billet aluminum. With that installed, all 302/351W SEFI components are usable on the 400. At the time, I was heavy into offroading and didn't want to mess around with a distributor anymore, so I went the extra yard and installed a 36-1 trigger wheel on the crank pulley, mounted the crank position sensor, and replaced the dizzy with a stock Ford cam sync. So with that I have Ford EDIS8 via coil packs and it is literally impervious to submersion. I would also note that all of these components were junkyard parts I sourced together over time, so the conversion was very economical.
I’ve installed Howell, msd atomic and Fitech on various trucks. I wouldn’t recommend any of them. Fitech has horrible customer service and they have lots of issues with software. Msd worked ok but In order to get it dialed you really needed an msd dizzy and msd box. Holley seems to be pretty good but I have no personal experience with it. Biggest issues I have with any throttle body efi system is if it/when it decides to take a dump you are stuck. (Usually at night 200 miles from home) No limp mode to get home.
I’d rather run a carb that I can field strip and rebuild if I get a load of bad gas or something else.