I run a Mega Squirt 3 Pro ecu in my mpfi slant six. It has been in use since 2016 and has been reliable. The intake is a Clifford 4 bbl unit that I drilled the NOS bosses to accept the fuel injectors. The injectors that I use are Ford 460 units. There are on line FI sizing calculators that can help you pick injectors.
For the Throttle Body I used a GM Vortec unit from a 4.3L v6 motor. I made an adapter from phenolic canvas to fit the TB to the intake.
The fuel tank and pump came from Tanks Inc. I made the delivery and return fuel lines.
I use a locked lean burn distributor with a MSD 6 box running through the MS3 Pro to control the ignition.
It runs well. I originally set this motor up for drag racing but have recently returned the car to street use and have done autocross with it.
An AEM O2 gauge provides the feedback.
A dual electric fan from a Ford Contour fits fine between the radiator and the water pump. There is a posting here on FABO that goes through the wiring.
I am a fan for FI for racing and performance, but I have my doubts that MPFI will provide sufficient fuel economy gains over a well tuned carb to overcome the added expense. But if you want to go that way, go for it.
Mega Squirt has a great online support forum and many users.
If I can prove any additional info to you on my install feel free to message me.
Some other engine / body ideas to consider that can affect fuel economy.
*work with the machine shop to get modern clearances on the rod and crank journals so you can run 0 or 5 weight synthetic motor oil.
*install a windage tray
*use modern pistons with thin low tension piston rings
*the thinner rods on slant six cast crank motors will have less rod bearing friction.
*raise the static compression to 8.5
*install a front air dam and limit - clean up the air flow through the front grill area. On my 68 Barracuda I ducted the grill so that the only air that goes through the grill go through the radiator.
*re consider the AC, driving with the windows down at highway speeds hurts fuel economy.