I have a 1985 GS that the frame rails have taken a beating. I would like to know
if I could put a U shapped channel over the existing channel and tack weld it in place? Will this cause any problems are is there a better way?

 

You could do what you are thinking no problem. I don't think you need very heavy guage channel to get the strength that the area offers. The loads going through that area are not that great. Think more side to side not up and down. The strut area gets torchered compared to this. Yes, its all connected somehow but the rockers do more for you than the channels under the floor, they support the cowl, kick panels then the A pillars/stut area.
I would remove the old stuff bend up some new parts close to stock design and weld them back on.

 

I wouldn't waste time on it.

 

Hey thanks for the input.

 

I have used 14 gauge sheet metal formed in to a channel and tack welded to the bottom of the chassis. It worked surprisingly well. I used in on a RX3 I was circle tracking. Cars take quite a bit of abuse. I did notice the chassis seemed stiffer when jacking up the car.

It is a messy and time consuming job. I presume your under chassis is taking street abuse. It is only time!