I'm afraid that what I found shows that Ivan was survived by at least one daughter.
Lt Colonel Ivan Guthrie died in 1964 not 1983. https://www.wikitree.com/wiki/Guthrie-2636 There are several secondary sources but I found no primary source so this could be checked on Scotlands People. I was able to find one newspaper source mentioning that died then and that afterwards the estate was administered by trustees which is worth researching. It is very probable that he left a will. https://www.nrscotland.gov.uk/research/guides/wills-and-testaments. His eldest daughter Moyra Irene Gutherie appears to have been his heir She died in 1984 . She married an army officer Mark Phillips in April 1944 at Wellington Barracks. She had two children. Her grandson (son of her daughter https://www.wikitree.com/wiki/Phillips-21798) was her heir. He died in 2020 at the age of 52. I haven't looked any further. The castle was first sold by 'the family' prior to Moyra's death, not 1985.
Ivan Guthrie died in 1964
'but in the time of Col. Ivan Douglas Guthrie, the last laird, who died in 1964... Since Col. Guthrie’s death the estate is administered by trustees
British Newspaper Archive, Aberdeen Press and Journal Saturday 15 August 1970
sale of castle in 1984 mentioning previous sale
US Oilman is new owner of Guthrie Castle GUTHRIE Castle, near Arbroath one of Scotland’s loveliest smaller castles and for centuries home of the Guthrie family has been sold to American oil millionaire, Mr Daniel Pena. Two years ago the Guthrie family sold the castle and grounds to property developer Mr Neil Rae who in turn now sold it to Mr Pena. Mr had been given approval Angus District Council to convert the castle and grounds into a holiday complex including the provision of 50 timesharing holiday villas. Mr Pena, however, intends to make the castle his Scottish home and will be developing the holiday complex.
British Newspaper Archives, Aberdeen Press and Journal-Tuesday 23 October 1984
Marriage of Moyra
AT THE GUARDS CHAPEL Lieutenant Mark Henry Philips, Coldstream Guards, elder son of Lieutenant-Colonel and Mrs. H. B. Philips. of Heybridge, Tean, Staffordshire was married at the Guards Chapel, Wellington Barracks. London to-day, to Miss Moyra Irene Guthrie, elder daughter of Colonel Ivan Guthrie of Guthrie. M.C and Mrs. Guthrie, of Guthrie Castle. Forfarshire.
British Newspaper Archives Saturday 15 April 1944
Interestingly Moyra was also at sometime known by the surname of Perry (another marriage?) In 1969 Moya Irene Guthrie widow of Henley on Thames 'abandoned the surname of Perry'
https://www.thegazette.co.uk/London/issue/44892/page/7102/data.pdf
Moyra Irene Guthrie of Guthrie's Grave is in Guthrie churchyard.
https://www.gravestonephotos.com/public/gravedetails.php?grave=750483
Succession; http://www.scotarmigers.net/ssagallery/displayimage.php?album=16&pos=88