I had a look at the Datsun Competition method and I'm not a fan. The square shoulder produced by machining them down is another engineering no no as there is virtually no radius to avoid stress raisers. It would probably be stronger if you undercut it with a radius believe it or not. I would recommend looking at modifying the box location or modifying Falcon rods. Les Collins might have the Falcon type rods on the shelf(?).

I'd avoid cutting threads at all costs.

Another consideration could be remounting the caster rod boxes (IIRC these are bolt on in 120Y) by drilling another set of mounting holes if they allow.

I would recommend buying adjustable caster rods from a reputable Datsun parts shop. I used re-threaded LJ Torana caster rods in my 1600, and whilst they never broke, re-threading by 'cutting' a thread rather than 'rolling' a thread is an engineering crime against nature. Cutting threads creates stress risers at the thread root, whereas 'rolling' a thread 'peens' the material it is forming. Virtually all proprietary bolts/threaded items have 'rolled' threads as they are strong, easy and quick to produce.

Extra caster via the radius rod will also distort you bottom LCA bush too, so be aware of accelerated wear and extra tension. Wheel base will also change slightly too as the wheel moves forward.

Note that some of the spherical rod end caster rod kits will alter the pivot point slightly, though this probably isn't really too significant in the scheme of things.

For bushed rods I personally like the re-shaped Falcon arms as these are beefy and only the bolt on part is re-forged to suit.

Lowering the car can also put a lot of strain and wear on the caster rod bushes so bump steer spacers between the strut and steering arm help out a lot getting things back in the original plane.

Base height is definitely the go if you can get inserts to suit your application. I nearly adapted some S13 base height adjustable coilovers for better preload and corner weighing setup, but the majority of shock inserts would blow during a long rally stage. 36mm Bilstein inserts will do quite a few rallies between rebuilds. With a passenger of similar weight corner weights a less of an issue, and preload should be 'OK' if springs are at least captive.

Use to run Wilwoods on a Sports Racer and noticed a fair bit of pad taper but I just machine the pads true after each event. Worth noting it was virtually cheaper to replace the caliper than rebuild them with new pistons! Cheap calipers but good value. Most have no dust seals though.
RX7 S4 are great calipers from limited experience and can be cheap to buy. Excellent pad choices.
AP do a budget caliper for circle track guys called SC320, but they are still US$390ea which is cheap by AP standards.

Thanks Nick, pictures sent. Commodore tubes were cut out, the stubs were bored out in the mill and lathe. I TIG welded them in the lathe to make sure they were aligned (I made sure not to earth through the lathe bearings).

The stubs were hard to machine around the old weld effected area.

The conversion allows the use of 36mm Bilstein Commodore inserts as opposed to 30mm inserts from vehicles like early VW Golf etc.

The bigger piston shocks don't overheat as easily as smaller piston shocks during a rally stage. Shocks are best if revalved such as those that Stuart Wilkins sells.

...having no end of grief trying to upload resized iPhone images....

Converted my 200b struts to take Commodore strut tubes. More pictures soon!

Quoted $2150 for MCA Blue series suspension (as to suit a 510, non coil over rear), including adapting to strut stubs. $300 less for DIY stut mods.

Stew Wilkins sells the Commodore Bilsteins re-valved to suit Datsun rally cars - though they are around $850 (front pair). Standard Commo Bilsteins are around $600. Then there is the strut tube mods etc so quite a bit of money and work. I think he also re-valves the Commo wagon rear shocks to suit also, though would have to check. Rears around $400 standard, $650 re-valved. BTW: Commodore Bilsteins have a comparatively large piston size for better heat soak.
The KYB combo is around $500 total, Z31 front, mono tube rear, rather palletable price. As always, the danger is building the worlds cheapest 120Y into the most expensive! (Car cost $38, it was actually $40 but I found $2 in the car).
Also got some disappointing news - my 40mm Dellorto's I bought very very cheap from an old Alfa enthusiast work mate are DHLA40"G" marked - G as in emission carb and a right royal pain to do anything with... They might be up for sale soon. Good for a Sud and that's about it.