Don't forget its not just re drilling the hubs, one would have to re drill the rear drums to match. But there is no room on the rear hubs to rotate them 45 degrees and drill a new set of holes, as there is already a large hole for getting to the half shaft bolts. One can then only drill at say 22.5 degrees which might be too short a distance between the two holes (old and new) and promote stress cracks leading to drum and hub failure. See the image below you can see the bolt access holes win the hub at 45 degree's
If however you did decide to stick with 101.6mm PCD and change the studs from 7/16th to 12mm Metric versions then wobble nuts could be used, (you cannot get 7/16th wobble nuts to my knowledge).
Alternative solutions if you so set on "I want 100mm as I get bored of my wheels", may involve using front hubs that are already 100mm (chevette) that would Involve changing all the brakes and custom steering arms (not sure if anyone has engineered all of that yet).
The rear will even be harder as although the Chevette axle is 100MM PCD and has the right ratios for the gearbox, its a totally different mounting design. So to move the whole axle over will involve custom props hafts, custom handbrakes welding new monte to the axle and modifying your exhaust.
PS most 100mm wheels look terrible on a Viva, modern wheels on a old car smell of escort wannabe syndrome