Kind of guessing, but could be a direct swap from the 3.9L to 318 TBI, if the connectors are the same. Since the same year engines, they might match (injectors, TPS, IAC). The 318 had 2 injectors. If the 3.9L is just one injector, you should be able to parallel the 2 wires to the 2nd injector, and hopefully won't overload the drive transistor.
I have used Holley Pro-jection (2 injectors) on my 65 Newport for years. I have tried both control boxes and think the early big box is best. Even better is probably the earliest "by MSD" boxes which have rugged Weatherpak connectors. A crimped wire pulled out of the connector on my later box which has prettier bulkhead connector, but flimsy. The little 2Di box can run hot and stop pulsing the injectors. In my case, once when sitting on the front seat in the sun. It also acted funny twice one morning (cool) at exactly the same spot on the freeway where there was road construction. I suspect a strong radio messed it up. Both times it apparently flooded the engine with gas, which then blew open the muffler. After getting beyond the site, it ran fine again. With both boxes, I always seem to fool with the knobs, even with O2 feedback, otherwise it idles too rich or lean.
Your factory fuel controller is probably better. A Pro-jection throttle body should work with that, if you can adapt the connectors. With a 318, you probably don't need IAC since I doubt the AC compressor will load the engine much. The Pro-jection solenoid opens the throttle more when cold, like 80's carburetors. The Pro-jection TBI was designed for a GM manifold (3 bolts). I don't know if it was ever factory, but Holley sold the Pro-jection throttle body alone as an upgrade to the GM factory TBI (Rochester?). The later have a different looking connector (translucent & open). I grabbed the two injectors from a Dodge TBI RAM Van (4-bolt Holley) last time at the JY, since they look identical to mine, with the same black "round top" connector that Pro-jection injectors use. The latter run ~$80 ea on ebay. I'll try to report back if my Pro-jection connector doesn't fit.
