How are you repairing steel with aluminium rods
Crusty sills are the easy job: replacements are about £50 per side, and are complete from the door seal to the lower edge. And the plastic covers mean that very little finishing is necessary.
Wheel arches aren't too bad, but do require a lot of time consuming and expensive finishing. Welding in properly shaped metal will require very little filler, not fibreglass.
Now the bad news: you need to look at other parts of the car.
The biggest problem is rot from under the driver's feet, through the bulkhead and along the chassis rails to the front panel. This is the start of digging away at the rot on the driver's side:
To repair those areas properly will require removal of the radiator, trans plumbing, subframe, alternator, steering gear together with a lot of fabrication. The other side is worse because of the air conditioning, battery tray, ABS and wiring.
These are MOT repairs in progress to get another couple of years out of the car:
(that needs to be rotated 90degrees anti-clockwise and is the steering box mount)
End of the driver's sill/inner wheel arch/floor/seatbelt mount: