I would say it's Stellarax. ISAAC says "my beautiful ceration" in 58. in next issue ISAAC says "first of a new breed" There's also a continuity in panels: last panel of 58 presents Isaac & Mystrious voice, first panel of 59 (not counting those with Captain marvel & drax) shows Stelarax talking with Isaac. elysius appears several panels later.
But if You add him (or if you believe it's Elysius) You should add that character as "Mysterious"

In 3256: Captain Marvel #58 (v1), on last page, last panel, ISAAC is talking to some "Mysterious-Off-Panel-Voice" which still qualifies as an appearance, right? The problem is, it is not 100% revealed who that voice is. It is most likely either Elysius or Stellarax (both appear talking to ISAAC in the next issue). Any thoughts?

Spectacular Spider Man 155 should be after ASM 320 because of spider man's thoughts in SSM 155 page 3 concerning the overheard phone conversation which occurred on the last page of ASM 320. it should also be before ASM 321 as in ASM 321 it is revealed to peter that Aunt May did not have heart problems.

SSM 156 and 157 should be moved along with SSM 155 as they are part of the same plot arc. so thats
ASM 320
SSM 155-157
ASM 321-325

ahh, ok.

I just double-checked. The a and b stories appear to be concurrent, and the b storyline is concluded in #325.

When moving punisher War journal 9 Shadowmasters 1-4 should also be moved to continue to be before it. although the series is a flashback and can technically be placed anywhere it makes more sense to read it before Punisher war Journal 9 and Punisher v2 24.

From my reading, it has them just getting off the quinjet after The Crossing Line arc in the a stories...

Does anyone know if 325 mentions stuff from the B stories as well as the stuff from the A story.

My copies are buried in a closet at the moment, but if memory serves, doesn't Avengers #325 continue the storyline in the "b" stories from the previous several issues? If so, shouldn't it be placed after them?

Uncanny X-men annual thirteen should be before the Bloodstone arc that covers cap 357b to 362a because of the footnote on page 20 of cap 358 and Diamondbacks thoughts on the same page. I went and glanced at UXM annual 13 and it seems heavily involved in some continuity that takes place after the position that Cap 358 currently occupies in the order. it would probably be easier to move The bloodstone arc forward to after the Atlantis Attacks arc.

363 and 364 can be safely moved to after Atlantis Attacks as well.

I did some further checking and Iron Man 39-40 cannot be moved after Incredible Hulk 140.
Incredible Hulk 140 is direct continuation of avengers 88. Avengers 89 takes place simultanously with Avengers 88. While they friends help hulk three remaining members of Avengers takes part in adventure which is first part of Kree/skrull wars. It is mentioned there it takes place in the same time.
Cap & all the other comes back in the middle of that story line but Falcon is not with them. So neither Iron Man 39-40 should be between avengers 88&89, nor there should be reason for falcon to appear with Avengers after Kree/Skrull wars.
In my opinion it should be left as it is