There isn't a single, universally accepted way to say "70th" in Scottish Gaelic because the language varies regionally. However, the most common approach would be to use the ordinal number. You'd likely combine the word for seventy (
seachdad) with the ordinal suffix for "th" which depends on the specific dialect and may not always be explicitly added.
So a reasonable approximation would be seachdadamh (seachdad + -amh). However, depending on the context and dialect, it might be expressed differently. It's best to consult a Gaelic speaker familiar with the specific region you're interested in for a definitive answer.