You would be far better off using feedback in the design, as you already have an opamp you wouldn't need to add one - simply use the one you already have.
A few extra points:
R4 seems very low at 100 ohms, at 24V output it will provide 250mA base current to the transistor - which seems rather high.
L1 - a 1H 5A inductor sounds expemsive, do you really need it?, it's not normally included in PSU's like this.
Output range!, at lower voltages, with high current, you are going to be dissipating a huge! amount of heat - the input is labelled 40V (presumably the AC rating of the transformer?). If we are conservative, and assume only 40V across the transistors - 40V*5A is 200W. If you really need to have this kind of range, try and arrange for lower incoming voltages from the transformer - you could do this by switching taps on the transformer, use a comparator fed from the output, and a relay to switch between windings.