Okay, here's what you might do. Take an 80" piece of that 17"-wide plank, and rip it into two pieces 8"x80". They'll be the long edges of the table. Cut two more pieces of the same plank so they're 24"x16". They (plus the ends of the 80" pieces) are the short edges of the table. There's a 24x48 hole in the middle of the wood for the soapstone.
The 16"x24" pieces may expand or contract a bit across the grain. (All wood does this.) You can fasten them firmly at the side closest to the stone, so all the expansion happens at the table edge. This results in a fixed-size hole for the stone. Or you could firmly fasten the 24" pieces near the edge of the table, so the hole for the stone may change a little in the 48" dimension. Arranging the grain the way I've suggested means that the 24" dimension of the hole for the stone will not change.