Due to SOC revisions in the past couple decades, comparing the SOC classification found in IPUMS-USA to the ISCO88 classification does not seem easy to perform. IPUMS-USA does provide a different variable that might be compared more directly: OCC. A Google search could be helpful in finding public documents by other researchers who have converted occupational codes to ISCO88. For example, using OCC from the 2000 Census, the Centre for Longitudinal Studies matched OCC to ISCO88 (see documents at bottom of linked page for conversion). Keep in mind that occupational codes vary based on the sample year.
Note: you can find the original Census codebooks here. You can find the 2000 Census occupation codes on page G-177 of the 2000 codebook.
I hope this helps.