In addition, that long drag link combined with the difference in angle with the radius arm will probably make for some interesting bump steer.
LOL.. so here's what I would do for you if you if you brought this to my shop,,,I would make a block just like you ask'd and put it on top of the mount you have, drill four holes in it , and make it long enough to to go on the cross member... I would take 1/2'' stock, use my plasma table to cut a block to fit with holes for the u bolts I would weld the block in and mount the spring on top of the mount like you ask'd and be done... it will work with the spring you have...and the shackle's... they will work as is.. but not 100% correct... but will work... to fine tune it you could get the correct spring and shackles... this is my RPU front... it's perfect. spring on top. rides and drives perfect,...LOL been down the freeway 1000's of miles through the years..
it has two leafs on it now and no reverse curvature, that was just a pic with one leaf as a reference.