I think the thinnest hub centric wheel spacer that you can get is 1.25". I ran the 1.25" spacers at first with 35 MTRs on these wheels too but I had a lot more back spacing to start with than you apparently do.
back spacing on my stock rims is more like 6.25", which is why I run the 1.75" spacers now with 37s. I'm right where I want to be now at 4.5" bs.
with the 1.25" spacers, that would put you at ~3.64" bs, which might be a bit much, depending on who you ask and your axle tubes.