Sealing the ductwork between furnace and A/C coils?

I had a new furnace installed last fall, and it turns out they didn't set the fan to the right level for my A/C so it was blowing the condensed water up into the ducts in the ceiling which was subsequently raining down into my basement. In trying to find the source of the problem, I had to remove some of the duct metal immediately downstream (right furnace term?) of the furnace and immediately upstream of the A/C coil unit.
The guys that installed the furnace used the funky three-fold metal joints on all sides but then sealed it all up with what *looks* like plain old silicone goo. The tub & tile type.
Now I asked at Home Depot and the one guy sounded very certain that that wasn't the stuff - that I needed some other type of construction-grade stuff. Said, "Oh no, you're dealing with *heat* and so you need this stuff." Had a 700 in the name. PL700 maybe? It was cheaper so I figured he wasn't trying to put one over on me. The tube he pointed to had no temperature rating on it, and while it looked like it would do the job moisture-wise, I didn't like the sounds of the descriptions; it implied it was going to be hard to get off if I ever needed it off.
So I bought the silicone anyways. Says it's rated to 400F which I seriously hope my vents aren't hitting.
Anyone know if this is the right stuff? Or if there's anything better?
Thanks.
J
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Jamie wrote:

Rest easy. The max plenum temp won't be over ~140F. The silicone may off-gas for a week or so but after that it's inert.
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Speedy Jim wrote:

Hey, that's great news. I'll finish that up this weekend then. Thank you!
J
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