Creating a large, deep sliding dovetail about 32" long.

All,
I am creating a island/table out of 5" thick reclaimed douglas fir. The top will be about 7' x 32" x 5". My idea is to turn it in to a trestle table with oak legs. I would like to attach the top of the two legs to the bottom of the table top with a big sliding dovetail. Any ideas? I have been thinking about this for 3 weeks and I don't know how I am going to do it (with any accuracy).
a) There isn't a dovetail bit big enough, right? b) How do I get my chizel in the middle? c) Even if I use my circular saw, I still need to chizel out the corners..
My only thought is circularr saw with a bar welded to a cheap chisel, any other ideas?
Matt
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timberframing slick will do the job nicely. A regular bench chisel should work if you use it perpendicular to the dovetail. I've done some pretty big sliding dovetail grooves by using a back saw to cut the sides, then a chisel to chop out the waste, and a router plane to clean out the bottom....
-JD

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