I think from your description you are refuring to the l;ater indipendent modles. The wider metal wheel arches are to acomodate the wider trake needed to allow the indipendent front suspention to fit. As for fitting these to an older car... The wider Indi arches are a solid part of the car. The front ones could probably be fitted, but you would need to change the whole wing. Not sure if this would marry up exactly as the grill is a diffent shap if memory serves. The rear arches would have to be cut out and new ones welded in. In all not a quick job to be done well.
Also unless you have very wide wheels fitted, on non-standard axles I think the Indi wheel arches would be far too wide for an F70/75. Personally I have had wide alloy wheels with 31/10.50-15's fitted on a standard Fourtrak(leafsprung), without any extra or extended wheelarches with no problems of any kind.
Legally you should not be able to see any tread when looking straight down the side of the car from above the wheel. Even if yoyu can see a little you are not likely to be stopped for it, though Mr Policeman might add it to your cars 'defects' if you were in an accident.
Given the choice I would remove the plastic wheel arches (if the metal underneeth is sound) as all they tend to do is trap wet, and rot out the steel underneeth.