That's basically the same idea as what I did to my '37 except I bought a bolt-in kit instead of making my own. I'm not sure about the exact distance between the spring and the frame without measuring but I don't think it's more than an inch.

My kit had the shackle sleeve and 2 mounting bolt sleeves welded to an angled piece that is bolted both horizontally and also vertically through the bottom of the frame rail. I had to drill a 1.25" hole it the frame rail for the shackle sleeve. The kit uses rubber bushings. What you're planning looks like it will work just as well and probably better!

What may look a little weird in the pics is the foot or so of 3/16 angle iron welded to the tail end of my frame rails..... My dad & I first built this car back when I was in college. He thought the end of the frame looked a little thin to be holding up springs so he beefed it up! It looks kinda like crap but it's been there for many years......in a perfect world, I would just buy a beautiful new frame but that's not in the cards at this juncture....