New floor pans

[BlueBull]

Hi guys. I bought a car from a non-serious company in des Moines. They told me the car body and floor pans was welded the correct way. Fooled again! I discovered serious rust on the floor pans especially on the passenger side. I'm really pissed off they being so irresponsible.

To the the case. The car paint work and outer body as fenders, doors, trunk are very good and I want to keep it. So my question is, can I fix this problem (maybe replace the whole floor pan) without mess with the rest?? Is it good practise to build a jig (- is this the correct word??) to maintain the measures during the replacement?

I appriciate the input.