All softwoods share much the same microscopic structure, so there's a surprisingly good correspondance between density and Young's modulus along the grain. Cross grain stiffness relates most closely to the angle of the annual rings. Density is pretty easy to measure, although, of course, the thinner the piece is the less precise your thickness measurment will be. It's also possible to find the density (well, actually, specific gravity) directly: float a piece end-up in water, and mark how far up the water line is. If the piece is rectangular, and the mark is, say, 35% of the way up from the bottom, the specific gravity is .35, and the density is 350 kg/cubic meter or about 29#/cubic foot. That's low, but not uncommon for cedar.
At that density, I'd expect it to have a Young's modulus along the grain of about 8000 MegaPascals (metric units). If I read the conversion table correctly, that's about 1,160,000 psi.
Since the relatinoship between density and Young's modulus is linear, knowing where the other end of the line is allows you to figure out any point in between. For softwoods Red spruce and sitka tend to hold down the other end, with densities around 500 kg/m^3 and young's modulus along the grain of about 16,000 MPa., or 2,320,000 psi.
Cedar usually has to be thicker than spruce, due to the lower density and Young's modulus. Since stiffness is the limiting factor, and stiffness goes as the cube of thickness, you'd need to make a cedar top from the low end of the density scale about 26% thicker than spruce from the high density end: 1.26^3= 2.00. In between stuff will be in between.. 
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