Without pictures, I'll assume your SLX Rossin has typical Italian geometry from the 80's, which means short chainstays and forks made for short reach brakes. I highly doubt a 650b x 38 tire will fit with much clearance at all, if any. The only way to know for sure is to do a fit check.
Post pix showing tire clearance in the chainstays and fork from different angles and I could do a more complete assessment.
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