I wouldn't make book on it.
The length dimension is the one that will get you into trouble.
Having said that, are these open on one side only? Like a full built in?
If so, deduct from each measurement, one and a half to two full inches for the true mattress size.
Why? You may inquire?
To give you room for the bedding..... sheets, blankets, bunk covers, that sort of thing.
I had a local company make me a new mattress for my semi and I forgot to deduct... it was a bear to make up!
A standard twin is 39 by 75, an extra long twin is 39 by 80.
Elizabeth in Iowa
The carpeting is gone! The carpeting is gone! Long live the cork floor!