Question: Is it ideal to give the final aliya of a Sefer to the baal korei, or is saying chazak a hefsek before his beracha?
Answer: The Avudraham (Tefillos Shabbos 57) suggests a reason for the minhag to say chazak. The Midrash teaches that when Hashem speaks to Yehoshua and tells him chazak ve’ematz, he was holding a Sefer Torah (See Rema OC 139:11; Aruch Hashulchan OC 139:15). Others compare this to the hadran that one recites upon completing a sefer (See Taamei Haminhagim 339).
R’ Moshe Stern (Baer Moshe 3:28:2) and R’ Shmuel Wosner (Shevet Halevi 7:202:2) note that the words chazak, chazak venischazek are aimed at the one who was called up to the Torah. It makes no sense, therefore, for him to recite these words. Doing so before the beracha, would certainly constitute a hefsek. If they do want to sayTop of FormBottom of Form it, they should do so after reciting the beracha.
R’ Ephraim Greenblatt (Rivevos Ephraim 2:98; 4:80) adds that for this reason, the gabbai should not call up the baal korei for this aliya, as he will not be able to respond with those words. He quotes R’ Yosef Shalom Elyashiv as saying that if the baal korei was given this aliya, he must not repeat the words, and just say the beracha after his aliya.
In conclusion, the one who receives the last aliya of the sefer should not say the words, chazak, chazak venischazek. It is best not to give this aliya to the baal korei.