440/500 Cranking issues

I cannot read your diagram. You grounding the battery right in the trunk? You might take a look at the body attachment point. And how did you ground your engine to the battery?

Many people insist you need a ground cable direct from battery to engine, but many others have used the body/ frame as well. You need at least one and better two big jumpers from engine block to body. NOT the K member which is bolted in.

What is the "thing" up near the starter? Is that a feed through? I cannot read that.

Bear in mind that with a factory starter solenoid (the one in the starter) the Ford solenoid, and a disconnect, you are now going through THREE hi current switches and that many more terminals. Take a jaundiced eye at all of them