.

The General Instruction of the Roman Missal says (as Erik P. already mentioned) in § 64: «The Sequence, which is optional except on Easter Sunday and on Pentecost Day, is sung before the Alleluia.»

However the 1974 Graduale Romanum (reproducing the rubrics from the Ordo Cantus Missae) says in § 8 (my translation): «The Sequence, if it be the case, is sung after the last Alleluia (...) ommitting the Amen at the end.» I believe this is the order you are to observe, since you are singing the Mass and even using Gregorian chant.

That is also the order observed in the Extraordinary Form, in which the following (taken from Psallite Sapienter § 144) is observed (as already mentioned by Michael O'Connor): «On Easter Sunday (...) the Alleluia is begun in the usual way, but after the verse alleluia is not repeated, and the Sequence is sung; the verses are alternated between two groups, both singing together the final Amen and alleluia.»

While you can do the same in the Ordinary Form, if you want to carefully observe what the 1974 Graduale prescribes you will have to sing the Alleluia, the verse, repeat the Alleluia, and only then sing the Sequence, ommitting the final Amen and of course the Alleluia that follows it in books for the Extraordinary Form. Actually books for the Ordinary Form no longer have the Amen or the Alleluia.

Transposing either the Alleluia or the Sequence so that the latter begins in the note where the former ends (as suggested by Michael O'Connor) is most reasonable. (A choir I belong to will sing it thus next Sunday.)

PS --- Concerning the intonation of the Alleluia (short vs. long), notice that:

* in the Extraordinary Form, the Alleluia «is intoned by the cantor(s) as far as the asterisk; there the symbol ij indicates that what they have just sung is to be repeated by the full choir, who then continue beyond the asterisk to conclude the word with the jubilus (the long melisma on the syllable “a”)» (Psallite Sapienter § 51);

* in the Ordinary Form, while you can do the same (which is often very practical), if you want to carefully observe what the 1974 Graduale prescribes, «the Alleluia with its neum is sung entirely by the cantors, and is then repeated by the choir. If it seems good, it may also be sung only once by everybody» (§ 7, my translation).

> I think the more recent instruction prevails.

It is reasonable to say so, though it can also be argued that when chant is used the rubrics from the Ordo Cantus Missae should be followed, since they are not superseded by the GIRM. There are people well versed in liturgical matters supporting both interpretations. I am not qualified to offer an authorative opinion!