Can you get a mobile mechanic to come by and cut the old ones off and tack weld the new ones on or replace with all thread? I am assuming you can't get access to the back of the old bolt head, and can't get the new ones in for the same reason. Possibly need some kind of expert help, it won't cost as much as messing it up yourself.
Judy & Ralph, w/Molly (the Westie) & Sadie (the Fourche Terrier)
2011 Ford F350 6.7 Lariat
2013 Infinity 3850RL