What is involved in enlarging a concrete garage door opening?
Assuming the lintel I see in the second picture travels over the block wall, it should be safe to cut the opening. I'm a contractor and if I could get a subcontractor to do it for $800 and guarantee it in writing, he'd be hired.  It better be a good straight cut I can frame to.  Be sure to calculate the proper rough opening you really need and add an inch.  It is easy to add shims, but cutting again is expensive.