The second issue is the Idss, whilst similar (actually within the tolerance of a 2N3819, which is pretty wide) it does give a different theoretical current for a given tail resistor.
It is easier to explain the solution to the pinout issue with a picture.
Step 1 - rotate the BF256B 90 degrees clockwise.
Step 2 - bend the gate lead down.
Step 3 - bend the source lead to the left.
Now to fix the current. If you keep the same value of tail resistor as you had for the 2N3819 the current will be too low, so you need to reduce the value of the tail resistor. I have found that you need to reduce it by approx 34%. So multiply the value by 0.66 to get the value right for the BF256B.
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