Hi Greg
It's been a year since I had all that stuff a part, but here's some thoughts.
I remember removing the shock without removing the lower bolt. Remove the top shock bolt, compress the shock, remove the lower retaining nut and washers. I remember working the compressed shock out.
Getting the shock out improves access but causes another problem. The suspension drops so that the u-joints bind and the drive shafts can't be easily turned. Lifting the upright a little with a bottle jack solves this.
Double check that there are not two stacked set screws in the holes. I have seen instances where set screws were stacked. Probably not the case here especially if you can turn the bolt.
Here comes the more interesting bit. There are two versions of this bolt, "stepped" and "straight". My car has one of each. I think the stepped version came first. It stepps down to a smaller diameter at the shock. The shock has a steel sleeve that mates to the bolt and washers at each side. The sleave and washers can sieze on the bolt preventing withdrawal. Getting the shock off first helps assess the situation. The rubber bottom shock mount may slide off the steel sleeve. On my car the washers are aluminum and may face one direction or could just be bad memory.
The "straight" version of the bolt is just that; there is no step down at the shock mount. I vaguely remember this also being difficult to disassemble. There was ridge or something due to the side washers on the shock. Some fine emery cloth and twisting finally freed the bolt.
So, first get the shock off then make sure the bolt can be turned then check out the shock mounting end of the bolt.
You probably don't want to hear this, but I've had other makes where this bolt was just completely siezed in place. The whole rear suspension was disassembled and the bolt drilled out and replaced.
I'm poking around under the Ace today. If I think of anything else, I'll forward
Peter S