Does the Jatco 4spd auto fit straight up to a rb20det? is it same housing etc? Also, does the Rb20det auto gearbox require its own comp to function?

Yes, the jatco fits right up to the rb20det. Use the rb30e's ringgear/flywheel and torque converter.... Just make sure you get the gearbox vacuum stuff done right, and also have a look on gweeds? website (which I forget the address of, do a search) and get the circuit that will allow your VL to have a working rev-counter. Rather important, or the jatco won't change gears properly. (I did this conversion a long time ago to my first VL)