that would be a lot more work, for less quality results.
The one piece trunk floor will slide in from the rear easily on a 68-70 Charger. The trunk floor welds to the rear valance, so that needs to be drilled apart anyway. It's simply a case of drilling the valance free from the tail panel and ends, and then it's off, leaving a nice wide opening for the new panel to slide right in.
Odds are, the rear valance and or valance corners and rear crossmember will have rust issues too, so this allows you to access, and repair or replace those pieces too.