68 Heater core Barracuda

When I purchased my 68 the owner had tried replacing the heater core. It has been out of the car for the 2-3 years I have had it and I finally have gotten around to doing it.

Here is the run down:

He removed the entire assembly and ordered a core but it didn't fit. Ordered a second, same thing, so he just left it as is knowing he was selling.

Here is what I found:

The two new cores basically fit, but the metal tabs on them are slightly too long and keep the box from fully closing. I figure I can file or bend them to get the box to close.

I do not think he gave me the clips the close the box. I saw some on ebay- any recommendations?

One of the new cores has foam on the two long sides of the core. I then saw there is a heater box insulation kit. Should I use one or does that not matter?

A piece of the box broke off, maybe 3" x 2". It fits back in perfectly. Should I specifically use a fiberglass mending product or will anything work?

Anything i should do to the box while it is out?

as always- Thank you for your help