For 500 pounds calves, you can usually figure 6 square feet per head so if it's a 6' wide trailer, you can have 1 calf per foot of length.
However, 1000 lb cow doesn't necessarily need 12 square feet because she is taller so your using more volume of the trailer (height).
On a 40 foot featherlite I'm assuming is 7' wide, you could possibly have 46 head of 500 pounders (23,000 of cattle) but with the empty weight of the trailer you blow the top out of the trailers GVWR.
If the trailer weighs 8000 + 23000 of cattle your at 31,000. If 20% of the weight is on the truck (6200) the weight on the trailer axles is down to 24,800 so if the GVW is based on the axle rating, you may be OK but it had better have something rolling on the ground besides 6 tires rated at 3000 pounds each like most trailers do.