sagging leaf springs give added flex which would be a bonus.. you might have tried taller shackles (which would have leveled it).. also, I have had a lot of buddies who have done add-a-leafs or the like, end up with squeaking coming from the leaf pack
I really like the ingenuity.. I actually admire the resourcefulness and the ambition.. in my eyes, you are on your way to becoming a master fabricator so keep it up!
But, if the squeaks do eventually come, look there first. Look into the shackle idea and consider a stronger rear sway-bar for the rear.. the torsion/twisting of the center of the sway-bar will better control body-roll that is brought on by soft leaf-packs. Maybe weld thin 1x1 channel iron onto the center on the cross-member of the rear sway-bar for added rigidity. Make sure that it is exactly in the middle for consistency while driving on the road and taking turns at higher speeds. If it isnt even, you would experience more body roll when turning one way, and the added potential for fishtailing when turning the other. I dont know if aftermarket "sport" style swaybars are available, like for road racing little cars, but look that up and read for fun.
Also, anticipate odd tire wear with your setup (or any other rear/potentially uneven suspension mod) and monitor.. initially, rotate frequently for best results.
just my initial thoughts..