When does a truck stop being the same truck? (factory stamped parts locations, etc..)

[79 silverado]

This will be an abbreviated version:

I've gotten ahold of our family's old truck, an 87 Suburban. Its tired, and seen better days - the rear sill the tailgate mounts to is rusted out, quarter panel has rust holes low, rear wheelwells are rusted out at the seams between inner and outer, outer rocker panels have some rust starting... you get the idea.

Firewall is nice including in the cowl seam, floors were undercoated new so real issues there, but I'm wondering... at what point does rebuilding the truck leave it not the same truck?

Just a rebody with vin swap is out of the question for me, that would NOT feel like the same truck anymore. Are there certain areas that could be saved off the body I have that had date stamps into the metal, partial vin stamps, etc and re-incorporated into a partial shell from a donor, maybe re-use my cowl/dash panels and hinge pillars?

I'm curious for input from anyone who has crawled around and either fully parted out several or even restored one of these trucks that could give some direction into what parts/pieces I could shift/re-use to help me "feel" like this is the same truck we brought home back before I got my license all those years ago.

I don't have immeasurable amounts of money to throw at it and I know that although new quarters can be had through a contact of mine, the pair of those alone will run about $5500... and that doesn't include all the other metal areas needing work or the labor costs for it to be done "right"...