LTD are awesome! The rules about suspension is, unless you have shopping trolley casters fixed to the tow bar, shares in Midas, and a kidney belt from a dump truck driver, don't lower it any more than 38 mm( 1.5 inches )! Shame, 'cause Herrods XG XR8 ute looked kick ass tough, and it was lowered 4.5 inches at the back with 18 inch rims!
What LTD and Fairlanes have is a bare minimum spring rate increase to hold them up out of the weeds! They were not much different to the XF Fairmont Ghia.
I'd suggest duplicating the XE ESP spring set-up, and use a similar (cheeper!) shockie with the same bump and rebound settings as the Bilsten shockies the coil sprung ESP's used. LTD's were not super heavy...there's well over 250-350 kgs of weight extra in a V8, 9 inched, FMX'd, cast iron 351 Fairlane/LTD. From memory some early six cylinder LTD's didn't even trip the 1600 kg barrier! Only 50-100 kgs heavier than an XE Fairmont Ghia!
ESP's sit about an inch oops, 25 mm lower than stock Falcons, so yours should sit about 30 mm lower. They are firm springs which allow the suspension to soak bumps, but keep wallow in check. They used 24 mm front stabiliser bars, yours may be 24 any way, but 22 mm was standard on Falcons on the front. Another option is the front XR8/6 springs, sway bar and shocks from an XH/XG.
As for the rear springs, you could use EB or later springs off an XR8/6 or something. I think the suspension was a carry over, it was just the ratio's and fittings that differed.
I've seen good original V8 front springs from Pre-1979 barge LTD's "cut" half a coil, used on the fronts with some success, but it has to be engineer approved and tested. Cut springs can work loose from the spring pearch, as they are supposed to have a 25 mm straight bit on the bottom. People are no longer with us who have dabbled with springs. I don't give a sh*t about impatient people, I use the right tools, and rip the springs out while my neighbours wonder what the heck I've been doing for the last 3 hours!
I used to work at a materials testing lab, and my mate was selling his Ford and wanted a good trade in, so he told me to rip out his Lovells front coils. I did, and replaced them with cut 1978 LTD V8 coils to get the stock 425 mm ride height (as measured from the centre of the wheel to the bottom of the centre of the guard). Then I got 120 ping for them!. It used to sit at 358 mm, which was too low for his 1980 beast on 14X8's. They had a wild rate on them... at full compression on our 10 tonne load tester they needed 800 kg to bring the coils together...twice as much as the 78 springs! The machine looked awesome though, lowered 2.5 inches, mags, 245's. Rode like a billy cart, too!
Vinyl is the go... I plan to buy a whole bunch of leather scraps and stuff them up under the seats of my wagon. The toughness of plastic, the smell of leather...