If Melchizedek was Yahshua, then that would negate Tanach prophecy about Yahshua.  The Orthodox Rabbis, who are the foremost authority, have always maintained that Melchizedek was Shem, son of Noach.

Restoration Scriptures, True Name Edition, Study Bible:

Beresheeth - Genesis 14

18 And Melech-Tzedek melech of Salem brought out lechem and wine: and he was the kohen of El-Elyon. 19 And he blessed him, and said, Blessed be Avram of El-Elyon, Possessor of the shamayim and earth: 20 And blessed be El-Elyon, who has delivered your enemies into your hand.  And he paid him the ma'aser of all.

Footnote:  Melech-Tzedek was Shem the first Melech-Tzadik in a order that YHWH established and in which Yahshua would later come to serve, as its eternal High Priest.

Hebrews 7

3  Neither his eema nor abba are recorded in the genealogies, without beginning of days, nor end of chayim but made like the Son of YHWH; who is the One that abides as a Kohen continually.

Footnote:  This simply means that the original Melech-Tzadik was Shem, as confirmed in the Scroll of Jasher.  Yahshua was born into that existing order and became Melech-Tzadik as Shem before Him (as seen in v.4 where Abraham pays Shem tithes).  In this verse the author states that both the original Shem and the One sworn into that order by the oath of YHWH, are not listed in the Levitical genealogies.  No record of any of their parents being from Lewi are recorded, since neither was a priest in the order of Lewi.  Neither Shem, nor Yahshua has beggining of days, nor end, or in that service.  Both however do have genealogies listed in Scripture.  Shem like Yahshua served in an eternal order and positionally he prefigured Yahshua, who serves in that same eternal order that can never pass away.

4 Now conside how great this man was, to whom even the patriarch Avraham gave the ma'aser of the best.

Footnote:  Shem son of Noach.

5  And truy they that are of the sons of Lewi, who receive the office of the Kohanim, have a mitzvah to take the ma'aser from the people according to the Torah, that is, from their Yisraelite brothers, even though they come out of the loins of Avraham:

6  But he whose descent is not recorded in their genealogies received the ma'aser from Avraham, and blessed him that had the promises.

Footnote:  Shem blessed Avraham.

7  And without any contradiction the lesser is blessed by the greater.

Footnote:  Shem was greater than Avraham.