I have little experience with M1 Carbines, and when I field stripped an improperly assembled
AMAC M1 carbine repro, I noticed these gouges in the recessed area of the Op rod/Slide where the bolt lug
travels. Is this normal? If not, what would cause this?

That is not normal.  Not hard enough metal at the contact point with the lug and oprod.

My guess is someone relieved it to get it to fit right because something was out of spec.

It looks like it peened its way to fitting because one or the other was not matching up.  Although I agree the steel the slide is made from might be a bit too soft, I would just keep an eye on it.  If it doesn't get any worse, I wouldn't worry about it.

I'm thinking the receiver it was on was a little wonky and the bolt didn't roll as far as it normally would.

Guys,
I haven't seen peeing/mushrooming that bad since getting rid of a couple Century Golanis(Galil clones) years ago.

Yeah, the metal on the slide might be too soft.This is after all an AMAC repro. There is little to no equal wear on the bolt's locking lug.
Think it's still safe to shoot?

If I were to replace the slide/op rod with a surplus GI, what should I go with? There's like 6 or 7 different slides out there with questionable
availability

I would go with the Type IV, V, or VI.( later M2) version.  Stay away from the Type III, as the joint at the arm/front connection is thinner and breakage can occur.  I would prefer the Type V or Type VI. The deflector on the top is reshaped to help deflect brass a bit differently.  The bolt cam on both is slightly modified to provide a slight delay in opening, to cut down spark/flame. Type Vs may(MAY, not all will be marked) be marked "7160091" and Type VIs may be marked "7161843".  You may find modified Type Vs that look like VIs, but will be marked with the Type V number.

Awesome info!
Where is the best place to find a IV, V, or VI Slide? Would these slides be drop in replacement parts on an AMAC(Iver Johnson) or would they need to be fitted?

There should be(SHOULD ) no fitting required.  They are supposed to be a drop in part. You can probably find the slides on Gunbroker, eBay, Sarco, Numrich, or a Gunshow.