Rear seat floor pan replacement

[SilverFrost73]

Been replacing some body panels on the 73 Cuda and I've noticed a potential issue.
I'm at the back removing the taillight panel for the new trunk pan/extensions install and I've noticed that the LR tire sticks out past the quarter panel about an inch farther than bit does on the PS.
After further inspection I've noticed the pumpkin on the rear differential is shifted to the left and not centered in the tunnel.

Could things have shifted when I removed and replaced the rear under seat pan?
I replaced it after replacing the DS front floor pan. Not sure if I should continue on removing the taillight panel and installing the trunk pan and extensions or not?
Any advice regarding this issue is appreciated.

[DeathProofCuda]

Have you checked the measurements for the spring perches on each side relative to the pumpkin?

[SilverFrost73]

Have you checked the measurements for the spring perches on each side relative to the pumpkin?

Thank you for your response @deathproofcuda. I have not measured that but I will shortly and report back.

[Jocigar]

Try taking some diagonal rail to rail measurements?

Just to mention, it is possible to replace trunk pan by removing rear cross-member.

[SilverFrost73]

I didn't realize that the pinion isn't centered in the 3rd member and is offset to the passenger side. So the yoke is centered with the tunnel.
The measurement from the edge of the 3rd member to the center of the leaf spring is 17 5/8" on both sides.

@Jocigar I'll take those diagonal measurements of the frame rails. Thanks for the recommendation.
As for installing the trunk pan I have to replace the taillight panel so that's why I removed it.