89 Suburban: rear air & rear wheelwell seam rust

[Electricduane]

Hi, 2 questions:

1) A piece of Styrofoam (1/2", estimate) is in between the roof sheetmetal and my rear AC unit (I just removed the headliber). Is this how it was done at the factory? Seems to me these days there might be a better insulating material than a cheap piece of Styrofoam. I am considering having polyurethane bed liner (line-x ir similar) sprayed on the interior of the roof. Would I need additional insulation in addition to that?

2) I found some rust bubbling under the seam sealer material (rubbery stuff) where the tub joins the rear wheelwells. Is this common on these vehicles? It appears that it began on the outside of that seam and ate his way through to the underside of the seam sealer. I assume the only proper repair will be to remove all the rust and have the seam re-welded?

Any advice or comments on the above would be appreciated.