There's another member in a thread around here who's having his bubbling back arches repaired. Sounds a top job, having old metal cut out, new arches put in.
It really depends how rusty the back arches really are...
You may be able to get some kind member to chop out some good arches on here, or buy off the shelf pattern parts and replace them. But as ever, the real cost is in the time, effort and skill of the person/s who have to do the job of fitting, welding, smoothing, painting etc... For two arches done to a high standard is costing fellow member £400, if memory serves.
Next route, as you say, plastic arch covers but (in my humble) is only like 'solving' an annoying engine rattle by turning the stereo up louder! But I appreciate this car may not be for sitting on grass at shows, but for driving.
To be honest, if not too bad you may we get away with a local repair which will cost you some wet n dry, cure-rust stuff, and a rattle can. Not a 100% perfect job, but get you by for a couple of years or more. Pics would help us determine
