If you don't have or don't need TCSA, then you can rearrange the wiring such that the 12v supply on the white/brown and yellow/purple goes to the gear lever switch first, then from there to the gearbox switch, and from there to the solenoid i.e. the original order of components. Then you can simply (accessibility aside) insert the sequencer circuit between the manual and gearbox switches and it will function as intended.
If you have TCSA but don't need it for emissions inspections then with the wiring rearranged as above the driver's switch will be controlling TCSA instead of the gearbox switch. To prevent that the simplest thing is to remove the TCSA solenoid from the distributor vacuum pipe, and join the two halves of the pipe together. This will give you vacuum advance in all gears instead of just 4th gear. Clausager says the TCSA system was provided to prevent 'surging' in the lower gears by restricting vacuum advance to 4th gear only, so if you make this change you will have to see if it gives you that problem. I'd recommend making this change before doing anything about the sequencer circuit, so if you do have 'surging' problems and decide you still need TCSA, you will have to think again about adding the sequencer. It is technically possible to add the sequencer, or different circuitry to achieve the same effect, with the gearbox and driver's switches in this order, but they involve adding circuitry and components to the solenoid side of the gearbox switch. And if any fault should develop that allows 12v to continue to be extended to the solenoid when the gearbox switch has opened, you could find yourself reversing with overdrive engaged and destroy the unit. Because of that I'm not prepared to suggest any of these other options.