Heatsinks of Tr2 and Tr3 transistors must be sufficiently large.
To archive R4,R5 and R6 values, we used some suitable resistors in parallel. For R4 and R5 two parallel connected 0.3ohm/5W resistors for each are used. To take 6A output, we connected two 0.22ohm/5W resistors in parallel, and to take 8A output we connected three 0.33ohm/5W resistors in parallel instead of R6 resistor.
If you change the marked component values properly you can get up to 14V output. For this situation you must disconnect R1,R2,C5,C6,C1,C2,D1,D2 components and connect the positive terminal of D3 directly to the rectified and stabilized supply.
TIP142 is a darlington transistor so you can not change it with other normal transistors.
For an example of the circuit operation; When a 0.68 ohm load was connected, we desired to take 8A current at 5.5V but voltage drop was about 0.18V. This means at 5.32V we got 7.8A and error was about %3.3. At the same condition, ripple voltage was less than 25V(rms).
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