This discussion seems to have happened half a dozen times in this thread.
To me it seems clear on Google satellite view that there's plenty of room to build a new bridge on a new alignment at Clarkston without demolishing any buildings.
Glasgow
www.google.co.uk
You'd go through this hoarding:
Find local businesses, view maps and get driving directions in Google Maps.
www.google.co.uk
My one concern is that the road over the bridge and down to the roundabout might have to be steep.
However I now think they will not do it that way. I think they will alter the bridge in its existing location. The roadworks website says "refurbishment/reconstruction of structure".
The roadworks website also has an item for 2 nights of work coming up on 3-4 March, "carry out trial pits at base of parapets."