You're right, it's pretty tight in there. On my '53 cab there's a clip on the firewall that's 1/2 way between the dimmer switch and the clutch pedal, and the harness clips in there then goes down to the top of the frame. I drilled a hole in the cab flange just above the frame rail and put a zip-tie there to keep it from moving around. Then it continues for a little ways on top of the fram then moves to the inside and is held by the factory clips the rest of the way back.

Are you using the factory-type headlight terminal blocks? If so they will mount on the inner fender panel up close to the radiator. The loom comes from the firewall, up the driver's side to the driver's side block, then up and over in front of the radiator to the other side. If you want a custom look and have enough length you could hide a lot of the wiring you would see on a restored truck.

As far as splicing, IMHO you can't beat a true Western Union splice, soldered, and covered with heat shrink. When done correctly the splice is about the same diameter as the original wires, will have excellent conductivity and be stronger than the original wire. There's info on the net on how to do them. Otherwise you can use butt-connectors and a GOOD splice tool that is capable of doing insulated and non-insulated crimps. There should be info on the net on the right and wrong way to do crimps as well.