The issue I have is a 1990 302 harness and ecm put into an 87 300 truck . Both manual trans. Therefore the c202 round firewall connector has who knows how many wires wrong . What is the best way to go about figuring this out . Ive found the pin out diagram for the 1990 302 engine side but can't find nothing for the 87 4.9 cab side

 

Use AllData at your public library (free)
I could get you the schematic and a pinout at my library in 10 minutes
I visit mine twice a week for information

 

My local library doesn't offer it.

 

That sucks
I've used it all over the country for free including on Maui
I'd complain to the library staff about it
They generally have an online resource for car repair
Maybe they have Mitchell on demand or something else that is similar