I personally would not cite statements to the effect that "the root of all good is the pleasure of the stomach" as an authentic statement of correct Epicurean doctrine.
Well, according to the cites for U409, it seems rather well attested, including a quote from a letter by Metrodorus himself (via Plutarch )
QuoteAnd are not Metrodorus's words something like to these when he writes to his brother thus: It is none of our business to preserve the Greeks, or to get them to bestow garlands upon us for our wit, but to eat well and drink good wine, Timocrates, so as not to offend but pleasure our stomachs. And he saith again, in some other place in the same epistles: How gay and how assured was I, when I had once learned of Epicurus the true way of gratifying my stomach; for, believe me, philosopher Timocrates, our prime good lies at the stomach.
ἦ γὰρ οὐ τούτοις ἔοικε τὰ Μητροδώρου πρὸς τὸν ἀδελφὸν γράφοντος; οὐδὲν δεῖ σῴζειν τοὺς; Ἕλληνας οὐδ᾽ ἐπὶ σοφίᾳ στεφάνων παρ᾽ αὐτῶν τυγχάνειν, ἀλλ᾽ ἐσθίειν καὶ πίνειν οἶνον, ὦ Τιμόκρατες, ἀβλαβῶς τῇ γαστρὶ καὶ κεχαρισμένως.’ καὶ πάλιν πού φησιν ἐν τοῖς αὐτοῖς γράμμασιν ὡς ‘καὶ ἐχάρην καὶ ἐθρασυνάμην, ὅτι ἔμαθον παρ᾽ Ἐπικούρου ὀρθῶς γαστρὶ χαρίζεσθαι.’ καὶ ‘περὶ γαστέρα γάρ, ὦ φυσιολόγε Τιμόκρατες, τἀγαθόν.’ ’
If we accept "direct" quotes from Cicero, should we not probably accept "direct" quotes from Plutarch?
The word used for "belly" is indeed γαστρὶ. From whence we get words like gastric, gastroenterology, etc. Cicero writes "when hunger and thirst are banished by food and drink, the mere fact of getting rid of those distresses brings pleasure as a result. So as a rule, the removal of pain causes pleasure to take its place." There's also VS33: The body cries out to not be hungry, not be thirsty, not be cold. Anyone who has these things, and who is confident of continuing to have them, can rival the gods for happiness. (NOTE: "body" σαρκὸς is used here instead of "belly" but the idea is the same as Metrodorus' letter) There's also U200 (emphasis added):
[ U200 ]
Porphyry, Letter to Marcella, 30, [p. 209, 7 Nauck]: Do not think it unnatural that when the flesh cries out for anything, the soul should cry out too. The cry of the flesh is, "Let me not hunger, or thirst, or shiver," and it’s hard for the soul to restrain these desires. And while it is difficult for the soul to prevent these things, it is dangerous to neglect nature which daily proclaims self-sufficiency to the soul via the flesh which is intimately bonded to it.
Seneca, Letters to Lucilius, 4.10: Let me share with you a saying which pleased me today. It, too, is culled from another man’s Garden: "Poverty, brought into conformity with the law of nature, is great wealth." Do you know what limits that law of nature ordains for us? Merely to avert hunger, thirst, and cold.
Clement of Alexandria, Miscellanies, II 21, p. 178.41: Epicurus, who held that happiness consists in not being hungry, nor thirsty, nor cold...
Cf. Cicero, Tusculan Disputations, V.35.102: Time would fail me should I wish to carry on about the cause of poverty; for the matter is evident and nature herself teaches us daily how few and how small her needs are, and how cheaply satisfied.
So, the idea of "pleasure of the belly" seems to me to be fairly well-attested within the philosophy.