I've know fellows who did like you're saying, and used all of the stock later model brake lines. They mounted the proportioning valve down by the radiator like the later truck originally used. It all looked very nice, and the lines didn't look like they had to be tweaked very much at all.
This also allows you to run the long style 73-87 front brake hoses. That's the main sticky when swapping these trucks to disc brakes, is working out the details on brake line routing and hoses. On mine, I wound up buying special conversion hoses that ECE sells to do the job. I wish I would have done exactly like you are doing yours
Here's what a nasty old junkyard stock 71-72 setup looks like....
And here's the S-10 proportioning valve I wound up using...
I still wound up building new brake lines from scratch twice and I'm still unhappy with them. I'm going to wind up redoing them again soon to get them lookng like I want them to.
You're on the right track...