It can be done with the car jacked up on Axel stands, but it's a prick of a job doing it upside down.
On a 240Z/early260Z, Full new 1.6mm Zincseal rails from the front of the footwells back to up under boot floor crossmember, is around the $1000 mark fitted & painted, if the car in not on a rotisserie.
Add another $500 for a post '75 260Z or 2+2 as the rails are a PITA to make & fit...
Robbie - get a light under the car & take some photo's of the rails.
Pictures speak a thousand words