Based on a few threads I've seen posted around here I think it should be around 73 Ohms Empty and 12 Ohms on full.
Or somewhere in that ballpark, they're not the most exacting instruments. 158 seems a bit outside based on the old threads I've seen. Assuming the normal gas gauge and not the special sender for the optional low fuel warning light or anything like that.
This might be correct for 2nd Gen. Mustangs, but the 1st Gens are 76/6. I just went through this. A 10 OHM will read less than full on a 65/66 with the 16 gal. tanks. The 22 gals. with the later gauge will fare better with the 76/10. I have a 76/10 on my shelf.