I too have a dash pad that sags. It's off the car now and I plan on making sheet metal stiffeners to slide inside where the dash brackets go. I think (hope) that will greatly help.
I believe they sag because the cavity inside the pad is too large. Add a little heat and droop it goes....

 

I decided just go with the repop even though I didn't care for the different texture. There was no way I could ever make the old sagging dash pad as straight as the repop and afterwards I was very happy with it. So I say replace it. Just my 2 cents.

My finished dash except for thw white face guage overlays.

 

There are shops that restore them, but here the one I found could not do the 67/68 due to some factor of the design being too hard to emulate/repair.... it was expensive for other years. I redyed my 'verts, it was in OK shape, went repro for the coupe, have not installed it yet.

 

I have epoxied wire mesh to mine to hold straight works fine