A few more thoughts.
If your goal is the 3.slow in your truck, then by swapping in the whole cowl harness from the 1990 you are doing this this hard way. I can see an advantage, but it will not be that simple. The 1990 will have items in the harness you will not need or want. You will need to adapt, as Dan said, this harness to your mini's items. Easiest way is to just cut, solder, and shrink tube the connectors. As Dan said, you can sometimes move the pins, but sometimes the pins will not match the later model connectors. Most of the older stuff is 1/4" spade and you can buy those (I did a bunch of those on my project).
Also, if you tear into the 1990 harness to remove those connectors you don't need you could simplify the harness to your truck. Many of the items (Dan mentioned the clock) can be teased out back to the splices in the circuit, and the circuit closed back up with solder and shrink tube.
Personally, while this would be slick, it's too much work for me. I would rather leave your harness in place, and tease out from the cowl harness the EFI bits making a dedicated EFI harness that runs off its own fuse. This is exactly what I did with my 3FE.