"The First Gospel" by Harold Riley is where I came across it. He seems quite scholarly to me (eg he quotes Greek words in places); there's a chapter at page 69 of the above preview describing what he thinks was in 'Proto-Matthew'.
But your claim was of a theory that Matthew originally ended at 28:8 - I can't seen anything in the link in support of this - perhaps it isn't in the available chapter.
But on that - his argument seems to be that Mark borrowed not from Matthew, but from some lost proto-Matthew. To which the easy repost is - well if there is a proto-Matthew, surely there will also be a proto-Mark. It seems again like cherry picking.
There must be texts or other forms of narrative that existed before the gospels arose and that were used by various gospel authors alone, or in combination with the earlier gospels. The most obvious being the elusive Q - but we don't have these so this is all conjecture. From what we do have I think there is a consensus around the following:
1. Mark likely came first, with John the latest.
2. What we have from extant versions of the texts is not the original (the so called autograph) and there is amply evidence for numerous major and minor changes to the texts in those earliest extant texts suggesting likely significant change from the autograph to what we consider to be the 'settled' gospel texts.