Social Security lump sum backdated to 2006

I have a client who was a US permanent resident since the 1970’s, and immigrated to Canada 27 years ago and became a Canadian Citizen in the early 90’s. Client is now 91 years old. In 2019, from advice of a friend, the client checked if they are entitled to Social Secuity Benefits and received a lump sum payment of over $45,000 US along with a statement detailing backpayment amounts per year from 2006 - 2019.

How do I approach this? Can I T1-ADJ the last 7 years, or even the last 13 (I am guessing with a snail mail request to the CRA) and claim income spread over those years (would this be without penalty?) or lump sum claim the whole amount in 2019? Or is there a different approach I should take

Any advice would be appreciated, or reference to a folio #

Try the T1198 but be sure to convert to CDN $ before entering. I suspect this will be reviewed now or later so as CRA will want to see the official document with the breakdown.