You are doing something I have wanted to do for a while: a 5.7 in an early cuda!
I'llshare my plan with you here: I would get the inner fenders as completely as you can from the valiant body, and mount them on a rolling jig at the right level and with the engine on a rolling stand, you could mock up the inner fenders by rolling it up, marking it, trimming a little, etc. Once you get it to fit between the inner fenders with them at the right width and height, make a template and cut the inner fenders on the cuda the same way. You can then custom fab some sections to replace the cut out areas that are convex to match the engine. Weld them in, grind it all smooth, paint it up, and Voila: Stock looking inner fenders!