275/60/15 sidewall clearance to leaf spring clamps- suggestions?

That’s without the spacer in the pictures?

Unfortunately you’ve discovered that the 8 1/4 is actually narrower than an A-body 8 3/4. The 4.5” of backspace on a 15x8” works well on an 8 3/4 with BBP axles, but with an 8 1/4 or 7 1/4 you need more like 4” of backspace.

You can move the clamps, but I think you’re stuck with the 3/8” spacers. Upgrade your wheels studs, run an high quality billet spacer and make sure you keep everything torqued.