I see this asked over and over. Can a one piece B body trunk floor be installed with the 1/4's on? Why yes it can! I do it all the time.
Here I will show how:

Our subject is a 68 Road Runner but this method works for any 68-70 B body including Chargers, and 70-74 E bodies too.

First off, most B bodies that need a trunkfloor also need the rear crossmember and bumper reinforcement plates as well. The layers of steel swell up over time with rust.

On this car I used AMD panels. The trunkfloor fit perfect. The crossmember and bumpers plates fit very well too, althought the crossmember did lack the recess for the frame rail tabs on it's top flange, but that is easily remedied with a hammer in about 2 minutes.

I've been restoring Mopars for 25 years. Panel replacement and rust repair are tedious jobs that many people seem to dread, but I love it!

Hats off to AMD for making these excellent panels!

On 68-70 Chargers this can be done without removing the rear crossmember providing the rear valance is off.