I was also going to mention checking out Chris Schwartz work on workbenches.
In the book The Anarchist Workbench he mentions making a bench out of solid slabs and the issues you might face. The book is available on his website as a PDF here. Mainly about getting a full slab that is solid enough throughout for the bench top. He specifically mentions soft spots and sponginess issues. But in his case it was a matter of how long the wood was left outside. Might not be much of a concern for you, depending on how dead the tree is I suppose.
In the article that is linked in the video that karpintero referenced he also mentions some of the difficulties of working full slabs with hand tools. Which is something to keep in mind. Definitely not going to be a one weekend project regardless of if it’s the full slab or not.
Might be worth seeing if there were a local shop that would square it up for you.
I think it really just comes down to how comfortable you are with taking on a large project like that.
Personally I was all excited to make a bench and then once I got to looking into it the more I realized if I started it I would probably never finish it so I never started it.
But it would be cool to see updates on what ever project you make. Good luck!
Fill it up with petroleum distillate and revulcanize my tires post haste!