Painless Wiring Vets- Couple Questions ??

[WinDancer]

Sat down with my just-delivered new 18 circuit Painless wiring harness this morning.

My truck is a 69 GMC 1/2 ton 2WD.

I have been reading every post here that showed up under a search for wiring. Most say to just follow the instruction EXACTLY and it will be simple.
Um-Hm.

The harness is broken down into four sections as I expected:
Dash
Engine
Rear
Extra door stuff

The first thing I notice is that all the wires are bundled for the first 18 inches. This is obviously meant to stay in place.

1. Is this made so that the dash stuff goes out into the engine well and then back into the cab somehow? Or should I just cut open the factory bundling so I can run the dash wiring up into the dash area inside the cab?

2. The instructions say to route the rear wiring down onto the 'floor pan'. Can't do that without cutting and redoing the factory bundle ties. I was told that our trucks have the rear wiring out into the engine well and then down the firewall and into the frame for the trip to the rear.

So should I just redo the bundle to that the wiring can go the correct direction from the fuse block or am I way too slow to do this myself?

Thanks for your input-