Connect 5" NPT (National Pipe Taper) to 3-1/2" NPT (National Pipe Taper)

Stepping a 5-8 NPT port down to a 3-1/2-8 NPT fitting uses a NPT (ASME B1.20.1) reducing bush: a male 5-8 NPT on the outside and a female 3-1/2-8 NPT inside, both to the same NPT form so they seal on the thread flanks with PTFE tape. The major diameter goes from 141.3 mm at 5-8 NPT to 101.6 mm at 3-1/2-8 NPT. This is a single stocked reducing part; the step is two nominal sizes.

Both ends are the same NPT standard, so the only thing that changes is the size; do not swap standards at the same time. If you need to cross to a different thread form as well, add the correct thread adapter rather than forcing a same-standard reducer to do two jobs.