In the NT Canon itself the Pauline Corpus is placed AFTER the Gospels and there is no known NT Codices where any Pauline letter is found before gMatthew and No Apologetic writer have placed the Pauline Corpus first.
Now, in a writing attributed to Origen, it will be claimed that the Pauline 14 Epistles were Last
http://www.bible-researcher.com/origen.html
From Origen's Homilies on Joshua, viii. 1. (about 240)
This work exists only in a Latin translation, probably by Rufinus (d. 410). Some scholars think that Rufinus has contributed to the passage. The latin text here is copied from the text given in Christopher Wordsworth's On the Canon of the Scriptures of the Old and New Testament, and on the Apocrypha (London: Francis & John Rivington, 1848), Appendix A, p. 8.
Veniens vero Dominus noster Jesus Christus, cujus ille prior filius Nave designabat adventum, misit sacerdotes Apostolos suos portantes tubas ductiles, praedicationis magnificam coelestemque doctrinam. Sacerdotali tuba primus in Evangelio suo Matthaeus increpuit, Marcus quoque, Lucas et Joannes, suis singulis tubis sacerdotalibus cecinerunt. Petrus etiam duabus epistolarum suarum personat tubis. Jacobus quoque et Judas. Addit nihilominus atque et Joannes tuba canere per epistolas suas et Apocalypsim, 4 et Lucas Apostolorum gesta describens. Novissime autem ille veniens, qui dixit: Puto autem nos Deus novissimos Apostolos ostendit, [1 Cor. 4:9] et in quatuordecim epistolarum suarum fulminans tubis, muros Jericho et omnes idololatriae machinas et philosophorum dogmata usque ad fundamenta dejecit.
So too our Lord, whose advent was typified by the son of Nun, when he came sent his apostles as priests bearing well-wrought trumpets. Matthew first sounded the priestly trumpet in his Gospel. Mark also, Luke and John, each gave forth a strain on their priestly trumpets. Peter moreover sounds loudly on the twofold trumpet of his epistles; and so also James and Jude.
Still the number is incomplete, and John gives forth the trumpet-sound in his epistles and Apocalypse; 4 and Luke while describing the acts of the apostles. Lastly however came he who said, I think that God hath set forth us Apostles last of all, [1 Cor. 4:9] and thundering on the fourteen trumpets of his epistles threw down even to the ground the walls of Jericho, that is to say all the instruments of idolatry and the doctrines of philosophers.
Origen even argued that Paul wrote the Epistle to the Hebrews.
Origen's De Principiis 2To show more clearly, however, what we mean, let us take the illustration employed by the Apostle Paul in the Epistle to the Hebrews...
Origen's De Principiis 1.2 The Apostle Paul says, that the only-begotten Son is the “image of the invisible God,” and “the first-born of every creature.” And when writing to the Hebrews, he says of Him that He is “the brightness of His glory, and the express image of His person.”
Origen has admitted that the Fourteen Epistles--the Pauline Corpus and the Epistle to the Hebrews were composed after the Gospels, after Acts of the Apostles, the Epistles of Peter, James, John, Jude and the Apocalypse of John.
Origen has admitted that all the Epistles of Paul and Hebrews were composed LAST in the Canon.
We have Apologetic writings that make no mention at all of the Pauline Corpus and Hebrews up to the mid 3rd century.