I have an 870 mag loop that I need to reattach to a barrel.

1. What solder should I buy, the 70ma or pa or whatever is OOS at brownells it is a paste. I figured paste would be more forgiving for a newbie like myself.

2. I'm capable of things like this, have oxy-acc torches at the ready just need some help selecting a solder.

Bonus points if I can get this stuff on Amazon because I'd like to attempt this weekend.  The stuff on there seems mixed in reviews mostly complaining about poor flow.  Help me avoid this?

Try a local welding supply shop.

This. They'll likely fix you up with some flavor of SafetySilv and some flux.

It's not a hard job.  Make sure everything in clean, get it hot enough that the solder flows and a tiny bit more.  

Note:
Careful of over heating.  

If you clamp it together, which I would, keep in mind the solder will shrink. But  don't over tighten the clamp or excess heat may dent the barrel.  

Put a heat sink of some sort, even a wet rag on the rib (if it's a vet rib barrel that is)   they are soldered on too.  Don't want it separating.  

Hot enough to move the solder, 1100 degrees will F up you factory bluing.  Be warned.  

It's a $40-80 job if you pay somebody else to do it.  Which may be the best option but onl if you have a smith you trust.  Many will say "sure" but they've never done it and don't know the problems they could create.  They aren't gunsmiths.  They're living a lie.  Don't buy into it.  

Cool thanks for the advice. Idk any gunsmiths, and I'm not afraid to mess anything up.  No vent rib, and I'm going to cerokote when done so here goes nothing.

If you're close to Charlotte, NC I have a tube of it in my gun parts cabinet

Negative, but if this is specific for gunsmithing what brand and details is it?

TIX solder and flux from Brownell.

Low melt point, strong bond.