You can fabricate the lower arms or you can probably source parts and build something up like Jack's upper arm from oval track supply houses. To fabricate the arms you'll need to buy some threaded bungs you weld in the tube to take the rod end bearings and a collar that accepts the type of ball joint ( Pinto / Mustang II style ) you will be using. There should be good pictures in Tom's build log.
You can also get things like this from oval track catalogs. I think you would grind off the tab on the bottom and weld in a rear tube. I'm not sure how these are used in stock cars, they are called strut lower arms.
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There are drawings for the arms in the SketchUp models that are not perfect but should help. You can make a jig for the arms on a small piece of plywood that you screw scrap blocks into to locate the arms and collars while you tack them together.
You can use the SketchUp drawings to help you understand and choose offset etc. I know you don't have much practice yet with that so if you run let me know some example wheel choices, I can show the geometry in the drawing. We have adjustable wheels etc for that. I think I have seen good prices on Mustang wheels from from Ford Motorsport that are OEM ( so cheap and good quality ). You may be able to get reasonable stock wheels at a junkyard etc. With your car at 1/3 the weight, any wheels are likely wide enough.
I suppose 15" are not so available anymore, they would have been cheap a few years ago. Offset will depend on diameter. More than 25 mm would be good. The issue with the Pinto/MII spindles is they have a log of offset and King pin inclination, so you get a lot of scrub radius if you are not careful. You would like 1" to 2" of scrub radius and less is better, so we should try to avoid 2" inches.
Have your spindles come in yet? When the parts are in your hand I can explain how to look at them and understand what's going on.